Earnings Per Common Share - Computations of Basic 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
Income available to common shareholders:
Net income	$ 1,408,306	$ 1,315,114	$ 1,079,667
Less: Preferred stock dividends	(72,734)	(81,270)	(81,270)
Net income available to common equity	1,335,572	1,233,844	998,397
Less: Income attributable to unvested stock-based compensation awards	(8,069)	(10,385)	(10,708)
Net income available to common shareholders	$ 1,327,503	$ 1,223,459	$ 987,689
Weighted-average shares outstanding:
Common shares outstanding (including common stock issuable) and unvested stock-based compensation awards	153,092	158,121	138,285
Less: Unvested stock-based compensation awards	(933)	(1,341)	(1,482)
Weighted-average shares outstanding	152,159	156,780	136,803
Basic earnings per common share	$ 8.72	$ 7.80	$ 7.22