Stockholders' Equity and Warrants - Additional Information (Detail) (USD $)	1 Months Ended		3 Months Ended		12 Months Ended
	Jan. 29, 2013	Feb. 25, 2009	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2011	Dec. 31, 2012
Stockholders Equity Note [Line Items]
Common stock, authorized			240,000,000			240,000,000
Common stock purchase price	$ 19.60	$ 1.69
Net proceed from sale of common stock	$ 310,000,000	$ 22,800,000	$ 310,037,000
Warrants issued		10,784,024
Exercise price of warrants		2.15
Warrants exercised				5,805,843	4,978,181
Proceeds from warrants exercised				12,482,000	10,700,000
Adjustment of additional paid-up capital up on exercise of warrants				$ 87,000,000
Common stock
Stockholders Equity Note [Line Items]
Shares of common stock sold	16,489,893	14,378,698