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		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Income available to common shareholders:
Net income	$ 293,462	$ 183,108	$ 733,305	$ 711,739
Less: Preferred stock dividends (a)	(13,363)	(14,079)	(40,088)	(31,761)
Amortization of preferred stock discount (a)	(2,033)	(1,848)	(5,921)	(18,132)
Net income available to common equity	278,066	167,181	687,296	661,846
Less: Income attributable to unvested stock-based compensation awards	(4,181)	(2,513)	(10,475)	(9,905)
Net income available to common shareholders	$ 273,885	$ 164,668	$ 676,821	$ 651,941
Weighted-average shares outstanding:
Common shares outstanding (including common stock issuable) and unvested stock-based compensation awards	127,741	126,478	127,449	123,855
Less: Unvested stock-based compensation awards	(1,922)	(1,903)	(1,939)	(1,850)
Weighted-average shares outstanding	125,819	124,575	125,510	122,005
Basic earnings per common share	$ 2.18	$ 1.32	$ 5.39	$ 5.34