Earnings Per Common Share - Computations of Diluted Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income available to common equity	$ 1,335,572	$ 1,233,844	$ 998,397
Less: Income attributable to unvested stock-based compensation awards	(8,055)	(10,363)	(10,673)
Net income available to common shareholders	$ 1,327,517	$ 1,223,481	$ 987,724
Adjusted weighted-average shares outstanding:
Common and unvested stock-based compensation awards	153,092	158,121	138,285
Less: Unvested stock-based compensation awards	(933)	(1,341)	(1,482)
Plus: Incremental shares from assumed conversion of stock-based compensation awards and warrants to purchase common stock	392	524	730
Adjusted weighted-average shares outstanding	152,551	157,304	137,533
Diluted earnings per common share	$ 8.70	$ 7.78	$ 7.18