Accumulated Other Comprehensive (Loss) Income - Changes in Accumulated Other Comprehensive (Loss) Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			$ 1,819.7	[1]	$ 1,792.7
Other comprehensive income (loss) before reclassifications			(56.1)		45.1
Amounts reclassified from accumulated other comprehensive income (loss)			2.4		(12.7)
Net current-period other comprehensive income (loss)			(150.5)
Other comprehensive income, net of tax	$ (21.7)	$ 18.5	(53.7)		32.4
Ending Balance	1,797.1	1,891.3	1,797.1		1,891.3
ASU 2016-01 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of adoption of ASU						$ 0.0
ASU 2018-02 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of adoption of ASU						0.0
Foreign Currency Translation Adjustments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(19.0)		(17.6)
Other comprehensive income (loss) before reclassifications			(4.0)		(0.6)
Amounts reclassified from accumulated other comprehensive income (loss)			0.0		0.0
Net current-period other comprehensive income (loss)			(4.0)
Other comprehensive income, net of tax					(0.6)
Ending Balance	(23.0)	(18.2)	(23.0)		(18.2)
Foreign Currency Translation Adjustments [Member] | ASU 2016-01 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of adoption of ASU						0.0
Foreign Currency Translation Adjustments [Member] | ASU 2018-02 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of adoption of ASU						0.0
Unrealized Holding Gains on Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			121.9		72.4
Other comprehensive income (loss) before reclassifications			(52.1)		45.7
Amounts reclassified from accumulated other comprehensive income (loss)	12.6	(1.7)	2.4		(12.7)
Net current-period other comprehensive income (loss)			(145.1)
Other comprehensive income, net of tax					33.0
Ending Balance	(23.2)	105.4	(23.2)		105.4
Unrealized Holding Gains on Securities [Member] | ASU 2016-01 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of adoption of ASU						(117.5)
Unrealized Holding Gains on Securities [Member] | ASU 2018-02 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of adoption of ASU						22.1
Defined Benefit Pension Plans [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(6.3)		(7.1)
Other comprehensive income (loss) before reclassifications			0.0		0.0
Amounts reclassified from accumulated other comprehensive income (loss)			0.0		0.0
Net current-period other comprehensive income (loss)			(1.4)
Other comprehensive income, net of tax					0.0
Ending Balance	(7.7)	(7.1)	(7.7)		(7.1)
Defined Benefit Pension Plans [Member] | ASU 2016-01 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of adoption of ASU						0.0
Defined Benefit Pension Plans [Member] | ASU 2018-02 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of adoption of ASU						(1.4)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			96.6		47.7
Other comprehensive income, net of tax			(53.7)		32.4
Ending Balance	$ (53.9)	$ 80.1	$ (53.9)		$ 80.1
AOCI Attributable to Parent [Member] | ASU 2016-01 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of adoption of ASU						(117.5)
AOCI Attributable to Parent [Member] | ASU 2018-02 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cumulative effect of adoption of ASU						$ 20.7

[1] Derived from audited consolidated financial statements.