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, 2013	Mar. 31, 2012
Income available to common shareholders:
Net income	$ 274,113	$ 206,463
Less: Preferred stock dividends	(13,363)	(13,363)
Amortization of preferred stock discount	(2,147)	(1,924)
Net income available to common equity	258,603	191,176
Less: Income attributable to unvested stock-based compensation awards	(3,524)	(2,940)
Net income available to common shareholders	$ 255,079	$ 188,236
Weighted-average shares outstanding:
Common shares outstanding (including common stock issuable) and unvested stock-based compensation awards	129,449	127,157
Less: Unvested stock-based compensation awards	(1,780)	(1,937)
Weighted-average shares outstanding	127,669	125,220
Basic earnings per common share	$ 2.00	$ 1.50