Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 1,897.9	$ 1,707.7	$ 1,857.8	$ 1,763.7
Other comprehensive income (loss) before reclassifications			(47.5)	43.6
Amounts reclassified from accumulated other comprehensive loss			(9.1)	(15.2)
Net current-period other comprehensive income (loss)			(56.6)	28.4
Ending Balance	1,887.6	1,848.7	1,887.6	1,848.7
Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance				(2.2)
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			(37.9)	(22.6)
Other comprehensive income (loss) before reclassifications			0.2	(12.3)
Amounts reclassified from accumulated other comprehensive loss			0.0	0.0
Net current-period other comprehensive income (loss)			0.2	(12.3)
Ending Balance	(37.7)	(34.9)	(37.7)	(34.9)
Foreign Currency Translation Adjustments | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance				0.0
Unrealized Holding Gains on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			105.1	33.5
Other comprehensive income (loss) before reclassifications			(49.2)	55.9
Amounts reclassified from accumulated other comprehensive loss	7.8	4.0	(9.1)	(15.2)
Net current-period other comprehensive income (loss)			(58.3)	40.7
Ending Balance	46.8	79.9	46.8	79.9
Unrealized Holding Gains on Securities | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance				5.7
Defined Benefit Pension Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			(8.6)	(8.1)
Other comprehensive income (loss) before reclassifications			1.5	0.0
Amounts reclassified from accumulated other comprehensive loss			0.0	0.0
Net current-period other comprehensive income (loss)			1.5	0.0
Ending Balance	(7.1)	(8.1)	(7.1)	(8.1)
Defined Benefit Pension Plans | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance				0.0
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	23.2	5.9	58.6	2.8
Ending Balance	$ 2.0	$ 36.9	$ 2.0	36.9
Accumulated Other Comprehensive Income (Loss) | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance				$ 5.7