Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 Debt Securities	Dec. 31, 2011 Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities realized gains	$ 0	$ 0.3	$ 0	$ 0.4
Unrealized gains (losses) on available-for-sale securities, before tax			2.0	2.8
Unrealized gains (losses) on available-for-sale securities, tax expense			(1.2)	(1.0)
Reclassification adjustment on available-for-sale securities, before tax				0.2
Securities in unrealized loss position for less than one year					0.4	10.9
Securities at fair value with no continuous losses					$ 28.6	$ 21.5