Investment Securities - Investment Securities (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized cost, current	$ 2,789.2	$ 1,536.1
Accumulated gross unrealized gain, current	1.5	0.2
Accumulated gross unrealized loss, current	149.6	141.1
Debt securities, current	2,641.1	1,395.3
Amortized cost, non-current	52.8	52.6
Accumulated gross unrealized gain, non-current	0.0	0.1
Accumulated gross unrealized loss, non-current	4.8	4.7
Fair Value	48.0	48.0
Amortized Cost	2,842.0	1,588.7
Total Unrealized Gains	1.5	0.3
Total Unrealized Losses	154.4	145.8
Fair Value	2,689.1	1,443.3
Executive deferred compensation plan trust	13.0	11.1
Debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost, non-current	15.1	15.2
Accumulated gross unrealized gain, non-current	0.0	0.1
Accumulated gross unrealized loss, non-current	0.9	0.7
Fair Value	14.2	14.5
U.S. treasury notes
Debt Securities, Available-for-sale [Line Items]
Amortized cost, current	405.8	405.7
Accumulated gross unrealized gain, current	0.0	0.0
Accumulated gross unrealized loss, current	40.1	41.7
Debt securities, current	365.7	364.1
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost, current	928.8	547.2
Accumulated gross unrealized gain, current	0.7	0.2
Accumulated gross unrealized loss, current	52.6	49.5
Debt securities, current	876.9	497.8
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost, current	69.8	53.0
Accumulated gross unrealized gain, current	0.1	0.0
Accumulated gross unrealized loss, current	6.2	8.0
Debt securities, current	63.7	45.0
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost, current	515.7	199.8
Accumulated gross unrealized gain, current	0.6	0.0
Accumulated gross unrealized loss, current	10.7	9.1
Debt securities, current	505.6	190.7
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost, current	869.1	330.4
Accumulated gross unrealized gain, current	0.1	0.0
Accumulated gross unrealized loss, current	40.0	32.7
Debt securities, current	829.2	297.7
Mutual fund
Debt Securities, Available-for-sale [Line Items]
Equity securities FV-NI cost, non-current	28.7	28.4
Accumulated gross unrealized gain, non-current	0.0	0.0
Accumulated gross unrealized loss, non-current	3.9	3.9
Fair Value	24.8	24.5
Pooled investment fund
Debt Securities, Available-for-sale [Line Items]
Equity securities FV-NI cost, non-current	9.0	9.0
Accumulated gross unrealized gain, non-current	0.0	0.0
Accumulated gross unrealized loss, non-current	0.0	0.0
Fair Value	$ 9.0	$ 9.0