Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 1,297.5	$ 1,232.9
Other comprehensive income before reclassifications	(10.8)	29.2
Amounts reclassified from accumulated other comprehensive loss	(5.2)	17.5
Net current-period other comprehensive income	(16.0)	46.7
Ending Balance	1,404.5	1,244.5
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	51.8	(305.1)
Ending Balance	35.8	(258.4)
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0.1	(4.2)
Other comprehensive income before reclassifications	(0.1)	0.4
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0
Net current-period other comprehensive income	(0.1)	0.4
Ending Balance	0.0	(3.8)
Unrealized Holding Gains (Losses) on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	51.1	(293.1)
Other comprehensive income before reclassifications	(10.7)	28.0
Amounts reclassified from accumulated other comprehensive loss	(5.2)	17.5
Net current-period other comprehensive income	(15.9)	45.5
Ending Balance	35.2	(247.6)
Defined Benefit Pension Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0.6	(7.8)
Other comprehensive income before reclassifications	0.0	0.8
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0
Net current-period other comprehensive income	0.0	0.8
Ending Balance	$ 0.6	$ (7.0)