Common Stock - Additional Information (Detail) - USD ($)		3 Months Ended											12 Months Ended
	May. 29, 2013	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Class Of Stock [Line Items]
Common stock, authorized shares		150,000,000				150,000,000							150,000,000	150,000,000
Common stock, par value		$ 0.001				$ 0.001							$ 0.001	$ 0.001
Common stock voting rights													Each holder of common stock has one vote in respect to each share of stock held on record for the election of directors and on all matters submitted to a vote of shareholders of the Company.
Special cash dividend declared per share of common stock	$ 1.25
Special cash dividend declared for common stock	$ 40,500,000
Dividend declaration date		Nov. 11, 2015	Aug. 10, 2015	May 11, 2015	Feb. 12, 2015	Nov. 10, 2014	Aug. 13, 2014	May 21, 2014	Feb. 27, 2014	Dec. 04, 2013	Aug. 27, 2013	May 29, 2013
Dividend payable date		Dec. 09, 2015	Sep. 07, 2015	Jun. 08, 2015	Mar. 12, 2015	Dec. 08, 2014	Sep. 24, 2014	Jun. 26, 2014	Mar. 27, 2014	Dec. 30, 2013	Sep. 27, 2013	Jun. 24, 2013
Special cash dividend and regular quarterly dividend payment for common stock										$ 6,000,000			$ 0	$ 0
Dividends paid		$ 20,200,000				$ 14,900,000							20,187,000	14,940,000	$ 45,922,000
Special cash dividends related to expected forfeitures of non-vested Shares													$ 0	$ 100,000	$ 600,000
2013 Special Dividend
Class Of Stock [Line Items]
Dividend declaration date															May 29, 2013
Dividend payable date															Jun. 28, 2013