Investment Securities - Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency (Details) - USD ($) $ in Millions	Sep. 30, 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	$ 385.8	$ 360.6
Unfunded Commitments	66.9	83.7
Private equity funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	246.1	249.5
Unfunded Commitments	66.9	83.7
Private equity funds | Aerospace/Defense/Government
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	140.9	152.4
Unfunded Commitments	40.7	49.3
Private equity funds | Financial services
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	101.7	93.4
Unfunded Commitments	24.0	32.0
Private equity funds | Real estate
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	3.5	3.7
Unfunded Commitments	2.2	2.4
Hedge funds
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]
Fair Value	139.7	111.1
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	106.7	51.9
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	33.0	59.2
Unfunded Commitments	$ 0.0	$ 0.0