Investment Securities - Fair Value of Hedge Funds Subject to Restrictions on Redemption Frequency (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Private equity funds
Investment securities
Fair Value	$ 248.6	$ 140.6
Unfunded Commitments	57.7	94.3
Hedge funds
Investment securities
Fair Value	64.3	57.2
Unfunded Commitments	0.0	0.0
Aerospace/Defense/Government | Private equity funds
Investment securities
Fair Value	157.2	59.4
Unfunded Commitments	17.4	37.5
Financial services | Private equity funds
Investment securities
Fair Value	87.5	77.1
Unfunded Commitments	37.8	54.3
Real estate | Private equity funds
Investment securities
Fair Value	3.9	4.1
Unfunded Commitments	2.5	2.5
Long/short equity financials and business services | Hedge funds
Investment securities
Fair Value	54.4	49.0
Unfunded Commitments	0.0	0.0
European small/mid cap | Hedge funds
Investment securities
Fair Value	9.9	8.2
Unfunded Commitments	0.0	0.0
Private equity funds and hedge funds
Investment securities
Fair Value	312.9	197.8
Unfunded Commitments	$ 57.7	$ 94.3