Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 1,735.2	$ 1,857.8
Other comprehensive loss before reclassifications	(375.3)	(47.5)
Amounts reclassified from accumulated other comprehensive loss	65.1	(9.1)
Net current-period other comprehensive income (loss)	(310.2)	(56.6)
Ending Balance	1,324.9	1,887.6
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(35.3)	(37.9)
Other comprehensive loss before reclassifications	(1.4)	0.2
Amounts reclassified from accumulated other comprehensive loss	31.8	0.0
Net current-period other comprehensive income (loss)	30.4	0.2
Ending Balance	(4.9)	(37.7)
Unrealized Holding Gains (Losses) on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	19.7	105.1
Other comprehensive loss before reclassifications	(373.9)	(49.2)
Amounts reclassified from accumulated other comprehensive loss	33.3	(9.1)
Net current-period other comprehensive income (loss)	(340.6)	(58.3)
Ending Balance	(320.9)	46.8
Defined Benefit Pension Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(7.1)	(8.6)
Other comprehensive loss before reclassifications	0.0	1.5
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0
Net current-period other comprehensive income (loss)	0.0	1.5
Ending Balance	(7.1)	(7.1)
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(22.7)	58.6
Ending Balance	$ (332.9)	$ 2.0