Investment Securities - Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Total private equity funds and hedge funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	$ 379.2	$ 360.7
Unfunded Commitments	78.4	83.7
Private equity funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	240.6	249.5
Unfunded Commitments	78.4	83.7
Private equity funds \| Aerospace/Defense/Government
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	140.3	152.4
Unfunded Commitments	48.0	49.3
Private equity funds \| Financial services
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	96.6	93.4
Unfunded Commitments	28.0	32.0
Private equity funds \| Real estate
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	3.7	3.7
Unfunded Commitments	2.4	2.4
Hedge funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	138.6	111.2
Unfunded Commitments	0.0	0.0
Hedge funds \| Long/short all cap global
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	80.6	52.0
Unfunded Commitments	0.0	0.0
Hedge funds \| Long/short equity financials and business services
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	58.0	59.2
Unfunded Commitments	$ 0.0	$ 0.0