Earnings per common share (Details) (USD $) In Thousands, except Share data	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Net income available to common shareholders
Net income	$ 322,358	$ 188,749	$ 528,631	$ 339,704
Less: Preferred stock dividends	(7,184)	(10,056)	(17,682)	(20,113)
Amortization of preferred stock discount	(13,531)	(2,605)	(16,284)	(5,162)
Net income available to common equity	301,643	176,088	494,665	314,429
Less : Income attributable to unvested stock-based compensation awards	(4,479)	(2,500)	(7,382)	(4,403)
Net income available to common shareholders	297,164	173,588	487,283	310,026
Weighted-average shares outstanding:
Common shares outstanding (including common stock issuable) and unvested stock-based compensation awards	124,035,000	119,756,000	122,522,000	119,550,000
Less: Unvested stock-based compensation awards	(1,854,000)	(1,702,000)	(1,823,000)	(1,640,000)
Weighted-average shares outstanding	122,181,000	118,054,000	120,699,000	117,910,000
Basic earnings per common share	$ 2.43	$ 1.47	$ 4.04	$ 2.63
Computations of diluted earnings per common share
Net income available to common equity	301,643	176,088	494,665	314,429
Less : Income attributable to unvested stock-based compensation awards	(4,464)	(2,491)	(7,357)	(4,392)
Net income available to common shareholders	$ 297,179	$ 173,597	$ 487,308	$ 310,037
Adjusted weighted-average shares outstanding:
Common and unvested stock-based compensation	124,035,000	119,756,000	122,522,000	119,550,000
Less: Unvested stock-based compensation awards	(1,854,000)	(1,702,000)	(1,823,000)	(1,640,000)
Plus: Incremental shares from assumed conversion of stock-based compensation awards and convertible preferred stock plus: Incremental shares from assumed conversion of stock-based compensation awards and convertible preferred stock	615,000	824,000	633,000	659,000
Adjusted weighted-average shares outstanding	122,796,000	118,878,000	121,332,000	118,569,000
Diluted earnings per common share	$ 2.42	$ 1.46	$ 4.02	$ 2.61
Earnings per common share (Textuals) [Abstract]
Antidilutive securities excluded from computation of earnings per share	10,100,000	10,700,000	10,300,000	11,300,000