Income (Loss) Per Common Share - Computations of Basic and Diluted (Loss) Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share Reconciliation [Abstract]
Net income (loss)		$ 48,539	$ (127,700)	$ 17,941	$ 111,961
Participating earnings	[1]	(590)	(58)	(224)	(1,251)
Basic net income (loss) attributed to common shareholders		47,949	(127,758)	17,717	110,710
Reallocation of participating earnings	[1]	2	0	0	1
Diluted net income (loss) attributed to common shareholders		$ 47,951	$ (127,758)	$ 17,717	$ 110,711
Net income (loss) per common share:
Basic		$ 0.19	$ (0.52)	$ 0.07	$ 0.45
Diluted		$ 0.19	$ (0.52)	$ 0.07	$ 0.45

[1] Restricted Stock Awards represent participating securities because they participate in nonforfeitable dividends or distributions with common equity owners. Income allocable to participating securities represents the distributed and undistributed earnings attributable to the participating securities. Participating securities, however, do not participate in undistributed net losses.