S000010826 [Member] Average Annual Total Returns	12 Months Ended	60 Months Ended	106 Months Ended	120 Months Ended	158 Months Ended	307 Months Ended	309 Months Ended
S000010826 [Member] Average Annual Total Returns	Dec. 31, 2025	Dec. 31, 2025	Dec. 31, 2025	Dec. 31, 2025	Dec. 31, 2025	Dec. 31, 2025	Dec. 31, 2025
S&P MidCap 400® Index (reflects no deductions for fees, expenses or taxes)
Prospectus [Line Items]
Average Annual Return, Percent	7.50%	9.12%		10.72%
Russell 3000® Index (reflects no deductions for fees, expenses or taxes)
Prospectus [Line Items]
Average Annual Return, Percent	17.15%	13.15%		14.29%
C000030054 [Member]
Prospectus [Line Items]
Average Annual Return, Percent	7.10%	8.64%		10.21%
Performance Inception Date						May 31, 2000
C000030054 [Member] \| After Taxes on Distributions
Prospectus [Line Items]
Average Annual Return, Percent	4.77%	5.78%		7.75%
Performance Inception Date						May 31, 2000
C000030054 [Member] \| After Taxes on Distributions and Sales
Prospectus [Line Items]
Average Annual Return, Percent	5.56%	6.32%		7.78%
Performance Inception Date						May 31, 2000
C000030055 [Member]
Prospectus [Line Items]
Average Annual Return, Percent	7.27%	8.90%		10.48%
Performance Inception Date							Mar. 31, 2000
C000122594 [Member]
Prospectus [Line Items]
Average Annual Return, Percent	7.32%	8.91%		10.49%
Performance Inception Date					Nov. 08, 2012
C000173200 [Member]
Prospectus [Line Items]
Average Annual Return, Percent	7.32%	8.91%		10.48%
Performance Inception Date			Mar. 01, 2017