Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 1,819.7	$ 1,792.7	$ 1,668.1
Other comprehensive income (loss) before reclassifications	(82.8)	77.1
Amounts reclassified from accumulated other comprehensive income (loss)	4.9	(28.2)
Net current-period other comprehensive income (loss)	(77.9)	48.9
Ending Balance	1,746.7	1,819.7	1,792.7
Foreign Currency Translation Adjustments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(19.0)	(17.6)
Other comprehensive income (loss) before reclassifications	(3.4)	(1.4)
Amounts reclassified from accumulated other comprehensive income (loss)	0.0	0.0
Net current-period other comprehensive income (loss)	(3.4)	(1.4)
Ending Balance	(22.4)	(19.0)	(17.6)
Foreign Currency Translation Adjustments [Member] | Accounting Standards Update 2016-01 [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU, net of taxes				$ 0.0
Foreign Currency Translation Adjustments [Member] | Accounting Standards Update 2018-02 [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU, net of taxes				0.0
Unrealized Holding Gains on Securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	121.9	72.4
Other comprehensive income (loss) before reclassifications	(80.4)	77.7
Amounts reclassified from accumulated other comprehensive income (loss)	4.9	(28.2)	(10.0)
Net current-period other comprehensive income (loss)	(75.5)	49.5
Ending Balance	(49.0)	121.9	72.4
Unrealized Holding Gains on Securities [Member] | Accounting Standards Update 2016-01 [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU, net of taxes				(117.5)
Unrealized Holding Gains on Securities [Member] | Accounting Standards Update 2018-02 [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU, net of taxes				22.1
Defined Benefit Pension Plans [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(6.3)	(7.1)
Other comprehensive income (loss) before reclassifications	1.0	0.8
Amounts reclassified from accumulated other comprehensive income (loss)	0.0	0.0
Net current-period other comprehensive income (loss)	1.0	0.8
Ending Balance	(6.7)	(6.3)	(7.1)
Defined Benefit Pension Plans [Member] | Accounting Standards Update 2016-01 [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU, net of taxes				0.0
Defined Benefit Pension Plans [Member] | Accounting Standards Update 2018-02 [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU, net of taxes				(1.4)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	96.6	47.7	11.5
Ending Balance	$ (78.1)	96.6	$ 47.7
AOCI Attributable to Parent [Member] | Accounting Standards Update 2016-01 [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU, net of taxes		(117.5)		(117.5)
AOCI Attributable to Parent [Member] | Accounting Standards Update 2018-02 [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of adoption of ASU, net of taxes		$ 20.7		$ 20.7