Computation of Basic and Diluted Earnings (Loss) Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income (loss) available to common shareholders	$ 441,580	$ 495,401	$ (47,156)
Less: Dividends paid on and earnings/(loss) allocated to participating securities	(3,554)	(3,795)	(3,357)
Earnings/(loss) applicable to common stock	$ 438,026	$ 491,606	$ (50,513)
Weighted average common shares outstanding	487,073,951	485,150,173	448,982,223
Basic earnings (loss) per common share	$ 0.90	$ 1.01	$ (0.11)
Potential dilutive common shares	0	0	0
Total shares for diluted earnings (loss) per common share computation	487,073,951	485,150,173	448,982,223
Diluted earnings (loss) per common share and common share equivalents	$ 0.90	$ 1.01	$ (0.11)