Earnings per common share - Computations of Diluted 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
Computations of diluted earnings per common share
Net income available to common equity	$ 278,066	$ 167,181	$ 687,296	$ 661,846
Less: Income attributable to unvested stock-based compensation awards	(4,170)	(2,510)	(10,454)	(9,880)
Net income available to common shareholders	$ 273,896	$ 164,671	$ 676,842	$ 651,966
Adjusted weighted-average shares outstanding:
Common 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)
Plus: Incremental shares from assumed conversion of stock-based compensation awards and convertible preferred stock	474	285	426	516
Adjusted weighted-average shares outstanding	126,292	124,860	125,936	122,521
Diluted earnings per common share	$ 2.17	$ 1.32	$ 5.37	$ 5.32