Investment Securities - Unrealized losses On Debt Securities (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Fair value, less than one year	$ 1,495.8	$ 664.4
Gross unrealized losses, less than one year	40.6	52.2
Fair value, one year or longer	1,002.5	716.1
Gross unrealized losses, one year or longer	109.9	89.7
Fair Value	2,498.3	1,380.5
Gross Unrealized Losses	150.5	141.8
U.S. treasury notes
Debt Securities, Available-for-sale [Line Items]
Fair value, less than one year	68.9	123.7
Gross unrealized losses, less than one year	3.9	12.5
Fair value, one year or longer	296.9	240.4
Gross unrealized losses, one year or longer	36.2	29.2
Fair Value	365.8	364.1
Gross Unrealized Losses	40.1	41.7
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than one year	391.6	196.9
Gross unrealized losses, less than one year	11.9	15.1
Fair value, one year or longer	411.0	289.9
Gross unrealized losses, one year or longer	40.7	34.4
Fair Value	802.6	486.8
Gross Unrealized Losses	52.6	49.5
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Fair value, less than one year	39.1	28.1
Gross unrealized losses, less than one year	1.0	3.8
Fair value, one year or longer	28.0	19.1
Gross unrealized losses, one year or longer	6.1	5.0
Fair Value	67.1	47.2
Gross Unrealized Losses	7.1	8.8
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than one year	321.1	117.7
Gross unrealized losses, less than one year	4.5	4.3
Fair value, one year or longer	123.7	70.2
Gross unrealized losses, one year or longer	6.2	4.8
Fair Value	444.8	187.9
Gross Unrealized Losses	10.7	9.1
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Fair value, less than one year	675.1	198.1
Gross unrealized losses, less than one year	19.3	16.4
Fair value, one year or longer	142.9	96.5
Gross unrealized losses, one year or longer	20.7	16.3
Fair Value	818.0	294.6
Gross Unrealized Losses	$ 40.0	$ 32.7