Fair Value Measurements - Fair Value Hierarchy - Additional Information (Detail) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Fair Value, Measurement Inputs, Disclosure [Line Items]
Net unrealized gain and loss related to securities, before tax	$ 100.6		$ 100.6		$ 77.5
Auction Rate
Fair Value, Measurement Inputs, Disclosure [Line Items]
Net unrealized gain and loss related to securities, before tax	0.2		0.2		10.8
Net unrealized gain and loss related to securities, net tax	0.1		0.1		6.8
Realized gains on securites	5.2	2.1	9.6	2.6
Auction Rate | Redemptions by Issuers
Fair Value, Measurement Inputs, Disclosure [Line Items]
Realized gains on securites	5.2	2.1	9.5	2.6
Auction Rate | Sales of Securities
Fair Value, Measurement Inputs, Disclosure [Line Items]
Realized gains on securites			$ 0.1