Earnings Per Common Share - Computations of Basic Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income available to common shareholders:
Net income	$ 348,927	$ 298,528
Less: Preferred stock dividends	(18,237)	(20,318)
Net income available to common equity	330,690	278,210
Less: Income attributable to unvested stock-based compensation awards	(2,128)	(2,466)
Net income available to common shareholders	$ 328,562	$ 275,744
Weighted-average shares outstanding:
Common shares outstanding (including common stock issuable) and unvested stock-based compensation awards	155,463	160,220
Less: Unvested stock-based compensation awards	(1,036)	(1,486)
Weighted-average shares outstanding	154,427	158,734
Basic earnings per common share	$ 2.13	$ 1.74