Investment Securities - Rollforward of Fair Value Measurements by Level (Details) - Level 3 - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 614.2	$ 654.0
Net realized and unrealized gains	117.3	275.6
Amortization/accretion	0.0	0.0
Purchases	225.4	151.5
Sales	(75.9)	(8.2)
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Ending balance	890.6	614.2
Common equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0.0	0.1
Net realized and unrealized gains		0.0
Amortization/accretion		0.0
Purchases		0.0
Sales		(0.1)
Transfers in		0.0
Transfers out		0.0
Ending balance		0.0
Ark | Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Purchases	$ 9.6
MediaAlpha | Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Effect of transfers		(458.7)
MediaAlpha | Common equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Effect of transfers		$ 0.0