Investments - Aggregate Fair Value, Amortized Cost Basis and Net Unrealized Gains and Losses of Available-for-Sale Investments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	$ 0	$ 114,806	$ 0
Amortized Cost	0	114,808	0
Net Unrealized Gains (Losses)	0	(2)	0
Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	0	80,445	0
Amortized Cost	0	80,446	0
Net Unrealized Gains (Losses)	0	(1)	0
Government Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	0	17,022	0
Amortized Cost	0	17,022	0
Net Unrealized Gains (Losses)	0	0	0
Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Aggregate Fair Value	0	17,339	0
Amortized Cost	0	17,340	0
Net Unrealized Gains (Losses)	$ 0	$ (1)	$ 0