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, 2018	Mar. 31, 2017
Income available to common shareholders:
Net income	$ 352,610	$ 348,927
Less: Preferred stock dividends	(18,130)	(18,237)
Net income available to common equity	334,480	330,690
Less: Income attributable to unvested stock-based compensation awards	(1,732)	(2,128)
Net income available to common shareholders	$ 332,748	$ 328,562
Weighted-average shares outstanding:
Common shares outstanding (including common stock issuable) and unvested stock-based compensation awards	149,470	155,463
Less: Unvested stock-based compensation awards	(782)	(1,036)
Weighted-average shares outstanding	148,688	154,427
Basic earnings per common share	$ 2.24	$ 2.13