Earnings per common share - Computations of Basic Earnings Per Common Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income available to common shareholders:
Net income	$ 1,029,498	$ 859,479	$ 736,161
Less: Preferred stock dividends	(53,450)	(45,839)	(40,225)
Amortization of preferred stock discount	(8,026)	(20,018)	(10,518)
Net income available to common equity	968,022	793,622	685,418
Less: Income attributable to unvested stock-based compensation awards	(14,632)	(11,879)	(9,592)
Net income available to common shareholders	$ 953,390	$ 781,743	$ 675,826
Weighted-average shares outstanding:
Common shares outstanding (including common stock issuable) and unvested stock-based compensation awards	127,793	124,524	119,852
Less: Unvested stock-based compensation awards	(1,929)	(1,861)	(1,661)
Weighted-average shares outstanding	125,864	122,663	118,191
Basic earnings per common share	$ 7.57	$ 6.37	$ 5.72