Earnings per common share (Details) (USD $) In Thousands, except Share data	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Income available to common shareholders:
Net income	$ 183,108	$ 192,015	$ 711,739	$ 531,719
Less: Preferred stock dividends	(14,079)	(10,056)	(31,761)	(30,169)
Amortization of preferred stock discount	(1,848)	(2,653)	(18,132)	(7,815)
Net income available to common equity	167,181	179,306	661,846	493,735
Less : Income attributable to unvested stock-based compensation awards	(2,513)	(2,526)	(9,905)	(6,924)
Net income available to common shareholders	164,668	176,780	651,941	486,811
Weighted-average shares outstanding:
Common shares outstanding (including common stock issuable) and unvested stock-based compensation awards	126,478,000	120,010,000	123,855,000	119,705,000
Less: Unvested stock-based compensation awards	(1,903,000)	(1,690,000)	(1,850,000)	(1,657,000)
Weighted-average shares outstanding	124,575,000	118,320,000	122,005,000	118,048,000
Basic earnings per common share	$ 1.32	$ 1.49	$ 5.34	$ 4.12
Computations of diluted earnings per common share
Net income available to common equity	167,181	179,306	661,846	493,735
Less : Income attributable to unvested stock-based compensation awards	(2,510)	(2,517)	(9,880)	(6,904)
Net income available to common stockholders	$ 164,671	$ 176,789	$ 651,966	$ 486,831
Adjusted weighted-average shares outstanding:
Common and unvested stock-based compensation awards	126,478,000	120,010,000	123,855,000	119,705,000
Less: Unvested stock-based compensation awards	(1,903,000)	(1,690,000)	(1,850,000)	(1,657,000)
Plus: Incremental shares from assumed conversion of stock-based compensation awards and convertible preferred stock	285,000	835,000	516,000	718,000
Adjusted weighted-average shares outstanding	124,860,000	119,155,000	122,521,000	118,766,000
Diluted earnings per common share	$ 1.32	$ 1.48	$ 5.32	$ 4.10
Earnings per common share (Textual) [Abstract]
Antidilutive securities excluded from computation of earnings per share	11,200,000	10,700,000	10,600,000	11,100,000