Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 1,763.7	$ 1,735.0	$ 1,812.9
Other comprehensive (loss) income before reclassifications	62.9	72.2
Amounts reclassified from accumulated other comprehensive income	(12.8)	8.7
Net current-period other comprehensive (loss) income	50.1	80.9
Ending Balance	1,857.8	1,763.7	1,735.0
Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(2.2)
Ending Balance		(2.2)
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(22.6)	(22.4)
Other comprehensive (loss) income before reclassifications	(15.3)	(0.2)
Amounts reclassified from accumulated other comprehensive income	0.0	0.0
Net current-period other comprehensive (loss) income	(15.3)	(0.2)
Ending Balance	(37.9)	(22.6)	(22.4)
Foreign Currency Translation Adjustments | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0.0
Ending Balance		0.0
Unrealized Gains and Losses on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	33.5	(49.0)
Other comprehensive (loss) income before reclassifications	78.7	73.8
Amounts reclassified from accumulated other comprehensive income	(12.8)	8.7	4.9
Net current-period other comprehensive (loss) income	65.9	82.5
Ending Balance	105.1	33.5	(49.0)
Unrealized Gains and Losses on Securities | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	5.7
Ending Balance		5.7
Defined Benefit Pension Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(8.1)	(6.7)
Other comprehensive (loss) income before reclassifications	(0.5)	(1.4)
Amounts reclassified from accumulated other comprehensive income	0.0	0.0
Net current-period other comprehensive (loss) income	(0.5)	(1.4)
Ending Balance	(8.6)	(8.1)	(6.7)
Defined Benefit Pension Plans | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	0.0
Ending Balance		0.0
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	2.8	(78.1)	96.6
Ending Balance	58.6	2.8	$ (78.1)
AOCI Attributable to Parent | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 5.7
Ending Balance		$ 5.7