Earnings Per Common Share - Computations of Basic Earnings Per Common Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income available to common shareholders:
Net income	$ 241,613	$ 229,017
Less: Preferred stock dividends	(20,318)	(14,674)
Net income available to common equity	221,295	214,343
Less: Income attributable to unvested stock-based compensation awards	(2,465)	(2,623)
Net income available to common shareholders	$ 218,830	$ 211,720
Weighted-average shares outstanding:
Common shares outstanding (including common stock issuable) and unvested stock-based compensation awards	133,542	131,800
Less: Unvested stock-based compensation awards	(1,493)	(1,588)
Weighted-average shares outstanding	132,049	130,212
Basic earnings per common share	$ 1.66	$ 1.63