Accumulated Other Comprehensive (Loss) Income - Changes in Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended			12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020		Jun. 30, 2019	Dec. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 1,636.8	$ 1,880.6	$ 1,781.1	[1]	$ 1,746.7	$ 1,746.7
Other comprehensive loss before reclassifications			16.6		82.8
Amounts reclassified from accumulated other comprehensive loss			(19.2)		4.2
Net current-period other comprehensive loss			$ (2.6)		87.0
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member			us-gaap:AccountingStandardsUpdate201613Member
Ending Balance	1,730.0	1,929.0	$ 1,730.0		1,929.0	$ 1,781.1	[1]
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)	(22.4)
Other comprehensive loss before reclassifications			(7.6)		(0.1)
Amounts reclassified from accumulated other comprehensive loss			0.0		0.0
Net current-period other comprehensive loss			(7.6)		(0.1)
Ending Balance	(30.2)	(22.5)	(30.2)		(22.5)	(22.6)
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 Holding Gains on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			33.5		(49.0)	(49.0)
Other comprehensive loss before reclassifications			24.2		82.9
Amounts reclassified from accumulated other comprehensive loss	(13.9)	1.6	(19.2)		4.2
Net current-period other comprehensive loss			5.0		87.1
Ending Balance	44.2	38.1	44.2		38.1	33.5
Unrealized Holding Gains 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)	(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)	(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
Ending Balance						0.0
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(101.9)	(27.6)	2.8		(78.1)	(78.1)
Ending Balance	$ 5.9	$ 8.9	5.9		$ 8.9	2.8
Accumulated Other Comprehensive Income (Loss) | Cumulative Effect, Period of Adoption, Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			$ 5.7
Ending Balance						$ 5.7

[1] Derived from audited consolidated financial statements.