Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended			12 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014		Jun. 30, 2013	Dec. 31, 2013
Accumulated Other Comprehensive Income Loss Net Of Tax [Abstract]
Beginning balance, Foreign currency translation adjustments			$ (11.1)		$ (11.5)	$ (11.5)
Other comprehensive income (loss) before reclassifications, Foreign currency translation adjustments					0	(0.4)
Amounts reclassified from accumulated other comprehensive income (loss), Foreign currency translation adjustments					(1.9)	0
Net current-period other comprehensive income (loss), Foreign currency translation adjustments					(1.9)	(0.4)
Ending balance, Foreign currency translation adjustments		(10.6)			(10.6)	(11.1)
Beginning balance, Unrealized holding gains on securities			190.1		163.9	163.9
Other comprehensive income (loss) before reclassifications, Unrealized holding gains on securities					(41.7)	39.3
Amounts reclassified from accumulated other comprehensive income (loss), Unrealized holding gains on securities	(13.1)	(9.5)	(13.1)		(10.3)	(13.1)
Net current-period other comprehensive income (loss), Unrealized holding gains on securities					(52.0)	26.2
Ending balance, Unrealized holding gains on securities		152.1			152.1	190.1
Beginning balance, Defined benefit pension plans			(4.6)		(4.6)	(4.6)
Other comprehensive income (loss) before reclassifications, Defined benefit pension plans					0	0
Amounts reclassified from accumulated other comprehensive income (loss), Defined benefit pension plans					0	0
Net current-period other comprehensive income (loss), Defined benefit pension plans					0	0
Ending balance, Defined benefit pension plans		(5.9)			(5.9)	(4.6)
Beginning balance			147.8	[1]	147.8	147.8
Other comprehensive income (loss) before reclassifications					(41.7)	39.7
Amounts reclassified from accumulated other comprehensive income (loss)					(12.2)	(13.1)
Other comprehensive income (loss), net of tax	25.5	(66.9)	26.6		(53.9)	26.6
Ending balance	$ 174.4	$ 135.6	$ 174.4		$ 135.6	$ 147.8	[1]

[1] Derived from audited consolidated financial statements.