Marketable Securities - Marketable Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 21,798	$ 36,680
Unrealized Gains	1	1
Unrealized Losses	(5)	(2)
Fair Value	21,794	36,679
Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,200	7,386
Unrealized Gains
Unrealized Losses	(5)	(2)
Fair Value	5,196	7,384
U.S. Treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,598	29,294
Unrealized Gains	1	1
Unrealized Losses
Fair Value	$ 16,598	$ 29,295