Marketable Securities - Additional Information (Detail) (USD $)	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2013 Securities	Jun. 30, 2013 Securities	Dec. 31, 2012 Securities
Amortized Cost And Fair Value Debt Securities [Abstract]
Realized gains or losses on marketable securities	$ 0	$ 0	$ 0
Investment maturity period		1 year	1 year
Debt securities in an unrealized loss position	22	22	30
Aggregate fair value of debt securities in an unrealized loss position	$ 8,000,000	$ 8,000,000	$ 13,700,000
Securities in significant unrealized loss position	0	0	0