Shareholder Report, Holdings (Details)	May 31, 2025
Class A \| Exploration & Production
Holdings [Line Items]
Percent of Net Asset Value	22.80%
Class A \| Midstream - Oil & Gas
Holdings [Line Items]
Percent of Net Asset Value	17.20%
Class A \| Integrated Oils
Holdings [Line Items]
Percent of Net Asset Value	16.80%
Class A \| Refining & Marketing
Holdings [Line Items]
Percent of Net Asset Value	9.60%
Class A \| Integrated Electric Utilities
Holdings [Line Items]
Percent of Net Asset Value	7.60%
Class A \| Power Generation
Holdings [Line Items]
Percent of Net Asset Value	5.90%
Class A \| Renewable Energy Equipment
Holdings [Line Items]
Percent of Net Asset Value	5.70%
Class A \| Oilfield Services & Equipment
Holdings [Line Items]
Percent of Net Asset Value	3.20%
Class A \| Base Metals
Holdings [Line Items]
Percent of Net Asset Value	2.90%
Class A \| Other (includes short-term Investments)
Holdings [Line Items]
Percent of Net Asset Value	8.30%
Class A \| UNITED STATES
Holdings [Line Items]
Percent of Net Asset Value	66.20%
Class A \| CANADA
Holdings [Line Items]
Percent of Net Asset Value	19.50%
Class A \| GERMANY
Holdings [Line Items]
Percent of Net Asset Value	4.40%
Class A \| DENMARK
Holdings [Line Items]
Percent of Net Asset Value	3.50%
Class A \| ITALY
Holdings [Line Items]
Percent of Net Asset Value	2.30%
Class A \| PORTUGAL
Holdings [Line Items]
Percent of Net Asset Value	2.10%
Class A \| NETHERLANDS
Holdings [Line Items]
Percent of Net Asset Value	1.30%
Class A \| Other (includes short-term Investments)
Holdings [Line Items]
Percent of Net Asset Value	0.70%
Class A \| Energy Transfer LP
Holdings [Line Items]
Percent of Net Asset Value	6.20%
Class A \| Hess Corp.
Holdings [Line Items]
Percent of Net Asset Value	6.20%
Class A \| TC Energy Corp.
Holdings [Line Items]
Percent of Net Asset Value	6.10%
Class A \| Exxon Mobil Corp.
Holdings [Line Items]
Percent of Net Asset Value	5.70%
Class A \| Marathon Petroleum Corp.
Holdings [Line Items]
Percent of Net Asset Value	5.40%
Class A \| ConocoPhillips
Holdings [Line Items]
Percent of Net Asset Value	5.20%
Class A \| Canadian Natural Resources Ltd.
Holdings [Line Items]
Percent of Net Asset Value	4.50%
Class A \| Phillips 66
Holdings [Line Items]
Percent of Net Asset Value	4.30%
Class A \| Shell PLC
Holdings [Line Items]
Percent of Net Asset Value	4.10%
Class A \| First Solar, Inc.
Holdings [Line Items]
Percent of Net Asset Value	3.60%
Class C \| Exploration & Production
Holdings [Line Items]
Percent of Net Asset Value	22.80%
Class C \| Midstream - Oil & Gas
Holdings [Line Items]
Percent of Net Asset Value	17.20%
Class C \| Integrated Oils
Holdings [Line Items]
Percent of Net Asset Value	16.80%
Class C \| Refining & Marketing
Holdings [Line Items]
Percent of Net Asset Value	9.60%
Class C \| Integrated Electric Utilities
Holdings [Line Items]
Percent of Net Asset Value	7.60%
Class C \| Power Generation
Holdings [Line Items]
Percent of Net Asset Value	5.90%
Class C \| Renewable Energy Equipment
Holdings [Line Items]
Percent of Net Asset Value	5.70%
Class C \| Oilfield Services & Equipment
Holdings [Line Items]
Percent of Net Asset Value	3.20%
Class C \| Base Metals
Holdings [Line Items]
Percent of Net Asset Value	2.90%
Class C \| Other (includes short-term Investments)
Holdings [Line Items]
Percent of Net Asset Value	8.30%
Class C \| UNITED STATES
Holdings [Line Items]
Percent of Net Asset Value	66.20%
Class C \| CANADA
Holdings [Line Items]
Percent of Net Asset Value	19.50%
Class C \| GERMANY
Holdings [Line Items]
Percent of Net Asset Value	4.40%
Class C \| DENMARK
Holdings [Line Items]
Percent of Net Asset Value	3.50%
Class C \| ITALY
Holdings [Line Items]
Percent of Net Asset Value	2.30%
Class C \| PORTUGAL
Holdings [Line Items]
Percent of Net Asset Value	2.10%
Class C \| NETHERLANDS
Holdings [Line Items]
Percent of Net Asset Value	1.30%
Class C \| Other (includes short-term Investments)
Holdings [Line Items]
Percent of Net Asset Value	0.70%
Class C \| Energy Transfer LP
Holdings [Line Items]
Percent of Net Asset Value	6.20%
Class C \| Hess Corp.
Holdings [Line Items]
Percent of Net Asset Value	6.20%
Class C \| TC Energy Corp.
Holdings [Line Items]
Percent of Net Asset Value	6.10%
Class C \| Exxon Mobil Corp.
Holdings [Line Items]
Percent of Net Asset Value	5.70%
Class C \| Marathon Petroleum Corp.
Holdings [Line Items]
Percent of Net Asset Value	5.40%
Class C \| ConocoPhillips
Holdings [Line Items]
Percent of Net Asset Value	5.20%
Class C \| Canadian Natural Resources Ltd.
Holdings [Line Items]
Percent of Net Asset Value	4.50%
Class C \| Phillips 66
Holdings [Line Items]
Percent of Net Asset Value	4.30%
Class C \| Shell PLC
Holdings [Line Items]
Percent of Net Asset Value	4.10%
Class C \| First Solar, Inc.
Holdings [Line Items]
Percent of Net Asset Value	3.60%
Class I \| Exploration & Production
Holdings [Line Items]
Percent of Net Asset Value	22.80%
Class I \| Midstream - Oil & Gas
Holdings [Line Items]
Percent of Net Asset Value	17.20%
Class I \| Integrated Oils
Holdings [Line Items]
Percent of Net Asset Value	16.80%
Class I \| Refining & Marketing
Holdings [Line Items]
Percent of Net Asset Value	9.60%
Class I \| Integrated Electric Utilities
Holdings [Line Items]
Percent of Net Asset Value	7.60%
Class I \| Power Generation
Holdings [Line Items]
Percent of Net Asset Value	5.90%
Class I \| Renewable Energy Equipment
Holdings [Line Items]
Percent of Net Asset Value	5.70%
Class I \| Oilfield Services & Equipment
Holdings [Line Items]
Percent of Net Asset Value	3.20%
Class I \| Base Metals
Holdings [Line Items]
Percent of Net Asset Value	2.90%
Class I \| Other (includes short-term Investments)
Holdings [Line Items]
Percent of Net Asset Value	8.30%
Class I \| UNITED STATES
Holdings [Line Items]
Percent of Net Asset Value	66.20%
Class I \| CANADA
Holdings [Line Items]
Percent of Net Asset Value	19.50%
Class I \| GERMANY
Holdings [Line Items]
Percent of Net Asset Value	4.40%
Class I \| DENMARK
Holdings [Line Items]
Percent of Net Asset Value	3.50%
Class I \| ITALY
Holdings [Line Items]
Percent of Net Asset Value	2.30%
Class I \| PORTUGAL
Holdings [Line Items]
Percent of Net Asset Value	2.10%
Class I \| NETHERLANDS
Holdings [Line Items]
Percent of Net Asset Value	1.30%
Class I \| Other (includes short-term Investments)
Holdings [Line Items]
Percent of Net Asset Value	0.70%
Class I \| Energy Transfer LP
Holdings [Line Items]
Percent of Net Asset Value	6.20%
Class I \| Hess Corp.
Holdings [Line Items]
Percent of Net Asset Value	6.20%
Class I \| TC Energy Corp.
Holdings [Line Items]
Percent of Net Asset Value	6.10%
Class I \| Exxon Mobil Corp.
Holdings [Line Items]
Percent of Net Asset Value	5.70%
Class I \| Marathon Petroleum Corp.
Holdings [Line Items]
Percent of Net Asset Value	5.40%
Class I \| ConocoPhillips
Holdings [Line Items]
Percent of Net Asset Value	5.20%
Class I \| Canadian Natural Resources Ltd.
Holdings [Line Items]
Percent of Net Asset Value	4.50%
Class I \| Phillips 66
Holdings [Line Items]
Percent of Net Asset Value	4.30%
Class I \| Shell PLC
Holdings [Line Items]
Percent of Net Asset Value	4.10%
Class I \| First Solar, Inc.
Holdings [Line Items]
Percent of Net Asset Value	3.60%
Class R \| Exploration & Production
Holdings [Line Items]
Percent of Net Asset Value	22.80%
Class R \| Midstream - Oil & Gas
Holdings [Line Items]
Percent of Net Asset Value	17.20%
Class R \| Integrated Oils
Holdings [Line Items]
Percent of Net Asset Value	16.80%
Class R \| Refining & Marketing
Holdings [Line Items]
Percent of Net Asset Value	9.60%
Class R \| Integrated Electric Utilities
Holdings [Line Items]
Percent of Net Asset Value	7.60%
Class R \| Power Generation
Holdings [Line Items]
Percent of Net Asset Value	5.90%
Class R \| Renewable Energy Equipment
Holdings [Line Items]
Percent of Net Asset Value	5.70%
Class R \| Oilfield Services & Equipment
Holdings [Line Items]
Percent of Net Asset Value	3.20%
Class R \| Base Metals
Holdings [Line Items]
Percent of Net Asset Value	2.90%
Class R \| Other (includes short-term Investments)
Holdings [Line Items]
Percent of Net Asset Value	8.30%
Class R \| UNITED STATES
Holdings [Line Items]
Percent of Net Asset Value	66.20%
Class R \| CANADA
Holdings [Line Items]
Percent of Net Asset Value	19.50%
Class R \| GERMANY
Holdings [Line Items]
Percent of Net Asset Value	4.40%
Class R \| DENMARK
Holdings [Line Items]
Percent of Net Asset Value	3.50%
Class R \| ITALY
Holdings [Line Items]
Percent of Net Asset Value	2.30%
Class R \| PORTUGAL
Holdings [Line Items]
Percent of Net Asset Value	2.10%
Class R \| NETHERLANDS
Holdings [Line Items]
Percent of Net Asset Value	1.30%
Class R \| Other (includes short-term Investments)
Holdings [Line Items]
Percent of Net Asset Value	0.70%
Class R \| Energy Transfer LP
Holdings [Line Items]
Percent of Net Asset Value	6.20%
Class R \| Hess Corp.
Holdings [Line Items]
Percent of Net Asset Value	6.20%
Class R \| TC Energy Corp.
Holdings [Line Items]
Percent of Net Asset Value	6.10%
Class R \| Exxon Mobil Corp.
Holdings [Line Items]
Percent of Net Asset Value	5.70%
Class R \| Marathon Petroleum Corp.
Holdings [Line Items]
Percent of Net Asset Value	5.40%
Class R \| ConocoPhillips
Holdings [Line Items]
Percent of Net Asset Value	5.20%
Class R \| Canadian Natural Resources Ltd.
Holdings [Line Items]
Percent of Net Asset Value	4.50%
Class R \| Phillips 66
Holdings [Line Items]
Percent of Net Asset Value	4.30%
Class R \| Shell PLC
Holdings [Line Items]
Percent of Net Asset Value	4.10%
Class R \| First Solar, Inc.
Holdings [Line Items]
Percent of Net Asset Value	3.60%
Class Z \| Exploration & Production
Holdings [Line Items]
Percent of Net Asset Value	22.80%
Class Z \| Midstream - Oil & Gas
Holdings [Line Items]
Percent of Net Asset Value	17.20%
Class Z \| Integrated Oils
Holdings [Line Items]
Percent of Net Asset Value	16.80%
Class Z \| Refining & Marketing
Holdings [Line Items]
Percent of Net Asset Value	9.60%
Class Z \| Integrated Electric Utilities
Holdings [Line Items]
Percent of Net Asset Value	7.60%
Class Z \| Power Generation
Holdings [Line Items]
Percent of Net Asset Value	5.90%
Class Z \| Renewable Energy Equipment
Holdings [Line Items]
Percent of Net Asset Value	5.70%
Class Z \| Oilfield Services & Equipment
Holdings [Line Items]
Percent of Net Asset Value	3.20%
Class Z \| Base Metals
Holdings [Line Items]
Percent of Net Asset Value	2.90%
Class Z \| Other (includes short-term Investments)
Holdings [Line Items]
Percent of Net Asset Value	8.30%
Class Z \| UNITED STATES
Holdings [Line Items]
Percent of Net Asset Value	66.20%
Class Z \| CANADA
Holdings [Line Items]
Percent of Net Asset Value	19.50%
Class Z \| GERMANY
Holdings [Line Items]
Percent of Net Asset Value	4.40%
Class Z \| DENMARK
Holdings [Line Items]
Percent of Net Asset Value	3.50%
Class Z \| ITALY
Holdings [Line Items]
Percent of Net Asset Value	2.30%
Class Z \| PORTUGAL
Holdings [Line Items]
Percent of Net Asset Value	2.10%
Class Z \| NETHERLANDS
Holdings [Line Items]
Percent of Net Asset Value	1.30%
Class Z \| Other (includes short-term Investments)
Holdings [Line Items]
Percent of Net Asset Value	0.70%
Class Z \| Energy Transfer LP
Holdings [Line Items]
Percent of Net Asset Value	6.20%
Class Z \| Hess Corp.
Holdings [Line Items]
Percent of Net Asset Value	6.20%
Class Z \| TC Energy Corp.
Holdings [Line Items]
Percent of Net Asset Value	6.10%
Class Z \| Exxon Mobil Corp.
Holdings [Line Items]
Percent of Net Asset Value	5.70%
Class Z \| Marathon Petroleum Corp.
Holdings [Line Items]
Percent of Net Asset Value	5.40%
Class Z \| ConocoPhillips
Holdings [Line Items]
Percent of Net Asset Value	5.20%
Class Z \| Canadian Natural Resources Ltd.
Holdings [Line Items]
Percent of Net Asset Value	4.50%
Class Z \| Phillips 66
Holdings [Line Items]
Percent of Net Asset Value	4.30%
Class Z \| Shell PLC
Holdings [Line Items]
Percent of Net Asset Value	4.10%
Class Z \| First Solar, Inc.
Holdings [Line Items]
Percent of Net Asset Value	3.60%

Shareholder Report, Holdings (Details)

May 31, 2025