Earnings Per Common Share - Computations of Basic Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income available to common shareholders:
Net income	$ 493,160	$ 381,053	$ 845,770	$ 729,980
Less: Preferred stock dividends	(18,130)	(18,237)	(36,260)	(36,474)
Net income available to common equity	475,030	362,816	809,510	693,506
Less: Income attributable to unvested stock-based compensation awards	(2,432)	(2,158)	(4,172)	(4,298)
Net income available to common shareholders	$ 472,598	$ 360,658	$ 805,338	$ 689,208
Weighted-average shares outstanding:
Common shares outstanding (including common stock issuable) and unvested stock-based compensation awards	145,571	153,770	147,510	154,612
Less: Unvested stock-based compensation awards	(746)	(913)	(764)	(974)
Weighted-average shares outstanding	144,825	152,857	146,746	153,638
Basic earnings per common share	$ 3.26	$ 2.36	$ 5.49	$ 4.49