Investments - Fair Values and Gross Unrealized Losses for all Available-for-Sale Securities in Unrealized Loss Position Deemed to be Temporary (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Unrealized Losses, Fair Value	$ 413.1	$ 144.2
Available-for-sale Securities, Unrealized Losses	(0.9)	(0.1)
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Unrealized Losses, Fair Value	251.6	144.2
Available-for-sale Securities, Unrealized Losses	(0.8)	(0.1)
U.S. government and agency debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Unrealized Losses, Fair Value	161.5
Available-for-sale Securities, Unrealized Losses	$ (0.1)