Other Operating Expense (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | 12 Months Ended | ||
|---|---|---|---|---|---|
Jun. 30, 2025 |
Jun. 30, 2024 |
Jun. 30, 2025 |
Jun. 30, 2024 |
Dec. 31, 2023 |
|
| Other Income and Expenses [Abstract] | |||||
| Business Promotion | $ 22.2 | $ 20.8 | $ 39.1 | $ 37.5 | |
| Staff Related | 7.0 | 8.2 | 11.2 | 22.4 | |
| FDIC Insurance Premiums | 9.5 | 9.8 | 17.4 | 28.5 | |
| Charitable Contributions | 3.1 | 74.0 | 5.0 | 76.0 | |
| Other Expenses | 49.1 | 62.5 | 102.2 | 111.3 | |
| Total Other Operating Expense | 90.9 | 175.3 | 174.9 | 275.7 | |
| Other Operating Expense [Line Items] | |||||
| Charitable Contributions | $ 3.1 | 74.0 | $ 5.0 | 76.0 | |
| Northern Trust Foundation | |||||
| Other Income and Expenses [Abstract] | |||||
| Charitable Contributions | 70.0 | 70.0 | |||
| Other Operating Expense [Line Items] | |||||
| Charitable Contributions | 70.0 | 70.0 | |||
| FDIC Special Assessment | |||||
| Other Operating Expense [Line Items] | |||||
| FDIC special assessment charge | $ 2.3 | $ 14.7 | $ 84.6 | ||