Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 1,232.9	$ 1,735.2
Other comprehensive income before reclassifications	29.2	(135.1)
Amounts reclassified from accumulated other comprehensive loss	17.5	22.9
Net current-period other comprehensive income (loss)	46.7	(112.2)
Ending Balance	1,244.5	1,610.8
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(305.1)	(22.7)
Ending Balance	(258.4)	(134.9)
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(4.2)	(35.3)
Other comprehensive income before reclassifications	0.4	4.1
Amounts reclassified from accumulated other comprehensive loss	0.0	27.3
Net current-period other comprehensive income (loss)	0.4	31.4
Ending Balance	(3.8)	(3.9)
Unrealized Holding Gains (Losses) on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(293.1)	19.7
Other comprehensive income before reclassifications	28.0	(139.2)
Amounts reclassified from accumulated other comprehensive loss	17.5	(4.4)
Net current-period other comprehensive income (loss)	45.5	(143.6)
Ending Balance	(247.6)	(123.9)
Defined Benefit Pension Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(7.8)	(7.1)
Other comprehensive income before reclassifications	0.8	0.0
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0
Net current-period other comprehensive income (loss)	0.8	0.0
Ending Balance	$ (7.0)	$ (7.1)