Earnings per common share (Details 1) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Computations of diluted earnings per common share
Net income available to common equity	$ 218,054	$ 301,643	$ 409,230	$ 494,665
Less: Income attributable to unvested stock based compensation awards	(3,338)	(4,464)	(6,272)	(7,357)
Net income available to common stockholders	$ 214,716	$ 297,179	$ 402,958	$ 487,308
Adjusted weighted-average shares outstanding:
Common and unvested stock-based compensation awards	127,445,000	124,035,000	127,301,000	122,522,000
Less: Unvested stock based compensation awards	(1,957,000)	(1,854,000)	(1,947,000)	(1,823,000)
Plus: Incremental shares from assumed conversion of stock-based compensation awards and convertible preferred stock	409,000	615,000	402,000	633,000
Adjusted weighted-average shares outstanding	125,897,000	122,796,000	125,756,000	121,332,000
Diluted earnings per common share	$ 1.71	$ 2.42	$ 3.20	$ 4.02
Earnings per common share (Textual) [Abstract]
Antidilutive securities excluded from computation of earnings per share	9,900,000	10,100,000	10,000,000	10,300,000