Accumulated Other Comprehensive (Loss) Income - Changes in Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020		Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 1,730.0	$ 1,929.0	$ 1,781.1	[1]	$ 1,746.7
Other comprehensive loss before reclassifications			43.6		76.1
Amounts reclassified from accumulated other comprehensive loss			(15.2)		6.7
Net current-period other comprehensive loss			$ 28.4		82.8
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member
Ending Balance	1,864.5	1,893.4	$ 1,864.5		1,893.4
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			(22.6)		(22.4)
Other comprehensive loss before reclassifications			(12.3)		(0.1)
Amounts reclassified from accumulated other comprehensive loss			0.0		0.0
Net current-period other comprehensive loss			(12.3)		(0.1)
Ending Balance	(34.9)	(22.5)	(34.9)		(22.5)
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			33.5		(49.0)
Other comprehensive loss before reclassifications			55.9		76.2
Amounts reclassified from accumulated other comprehensive loss	4.0	2.5	(15.2)		6.7
Net current-period other comprehensive loss			40.7		82.9
Ending Balance	79.9	33.9	79.9		33.9
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			(8.1)		(6.7)
Other comprehensive loss before reclassifications			0.0		0.0
Amounts reclassified from accumulated other comprehensive loss			0.0		0.0
Net current-period other comprehensive loss			0.0		0.0
Ending Balance	(8.1)	(6.7)	(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
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	5.9	8.9	2.8		(78.1)
Ending Balance	$ 36.9	$ 4.7	36.9		$ 4.7
Accumulated Other Comprehensive Income (Loss) | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			$ 5.7

[1] Derived from audited consolidated financial statements.