Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017		Jun. 30, 2016
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Beginning Balance			$ 47.7	[1]	$ 11.5
Other comprehensive income (loss) before reclassifications			45.1		61.8
Amounts reclassified from accumulated other comprehensive income (loss)			(12.7)		(4.2)
Other comprehensive income, net of tax	$ 18.5	$ 28.8	32.4		57.6
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Ending Balance	80.1	69.1	80.1		69.1
Foreign Currency Translation Adjustments [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Beginning Balance			(17.6)		(21.6)
Other comprehensive income (loss) before reclassifications			(0.6)		3.8
Amounts reclassified from accumulated other comprehensive income (loss)			0.0		0.0
Other comprehensive income, net of tax			(0.6)		3.8
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Ending Balance	(18.2)	(17.8)	(18.2)		(17.8)
Unrealized Holding Gains on Securities [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Beginning Balance			72.4		40.0
Other comprehensive income (loss) before reclassifications			45.7		58.0
Amounts reclassified from accumulated other comprehensive income (loss)			(12.7)		(4.2)
Other comprehensive income, net of tax			33.0		53.8
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Ending Balance	105.4	93.8	105.4		93.8
Defined Benefit Pension Plans [Member]
Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Beginning Balance			(7.1)		(6.9)
Other comprehensive income (loss) before reclassifications			0.0		0.0
Amounts reclassified from accumulated other comprehensive income (loss)			0.0		0.0
Other comprehensive income, net of tax			0.0		0.0
Accumulated Other Comprehensive Income (Loss) Net Of Tax, Ending Balance	$ (7.1)	$ (6.9)	$ (7.1)		$ (6.9)

[1] Derived from audited consolidated financial statements.