Earnings per common share - Computations of Diluted Earnings Per Common Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Computations of diluted earnings per common share
Net income available to common equity	$ 968,022	$ 793,622	$ 685,418
Less: Income attributable to unvested stock-based compensation awards	(14,593)	(11,857)	(9,565)
Net income available to common shareholders	$ 953,429	$ 781,765	$ 675,853
Adjusted weighted-average shares outstanding:
Common and unvested stock-based compensation awards	127,793	124,524	119,852
Less: Unvested stock-based compensation awards	(1,929)	(1,861)	(1,661)
Plus: Incremental shares from assumed conversion of stock-based compensation awards and convertible preferred stock	541	416	652
Adjusted weighted-average shares outstanding	126,405	123,079	118,843
Diluted earnings per common share	$ 7.54	$ 6.35	$ 5.69

Common shares outstanding (including common stock issuable) and unvested stock-based compensation awards.

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Income attributable to unvested Stock Based compensation awards diluted.

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Net Income (Loss) Available to Common Stockholders Diluted before Unvested Stock-Based Compensation Awards, Adjustment.

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Unvested stock-based compensation awards.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Net Income or Loss Available to Common Stockholders plus adjustments resulting from the assumption that dilutive convertible securities were converted, options or warrants were exercised, or that other shares were issued upon the satisfaction of certain conditions.

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The sum of dilutive potential common shares or units used in the calculation of the diluted per-share or per-unit computation.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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