Earnings per common share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Income available to common shareholders:
Net income	$ 233,380	$ 322,358	$ 439,843	$ 528,631
Less: Preferred stock dividends	(13,362)	(7,184)	(26,725)	(17,682)
Amortization of preferred stock discount	(1,964)	(13,531)	(3,888)	(16,284)
Net income available to common equity	218,054	301,643	409,230	494,665
Less: Income attributable to unvested stock based compensation awards	(3,345)	(4,479)	(6,283)	(7,382)
Net income available to common shareholders	$ 214,709	$ 297,164	$ 402,947	$ 487,283
Weighted-average shares outstanding:
Common shares outstanding (including common stock issuable) and unvested stock-based compensation awards	127,445	124,035	127,301	122,522
Less: Unvested stock based compensation awards	(1,957)	(1,854)	(1,947)	(1,823)
Weighted-average shares outstanding	125,488	122,181	125,354	120,699
Basic earnings per common share	$ 1.71	$ 2.43	$ 3.21	$ 4.04