Computation of Basic and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 128,798		$ 119,750		$ 378,263		$ 362,385
Less: Dividends paid on and earnings allocated to participating securities	(1,213)		(914)		(3,552)		(2,732)
Earnings applicable to common stock	$ 127,585		$ 118,836		$ 374,711		$ 359,653
Weighted average common shares outstanding	437,787,688		436,243,926		437,692,411		435,980,390
Basic earnings per common share	$ 0.29		$ 0.27		$ 0.86		$ 0.82
Potential dilutive common shares	5,664	[1]	33,640	[1]	5,026	[1]	371,359	[1]
Total shares for diluted earnings per share computation	437,793,352		436,277,566		437,697,437		436,351,749
Diluted earnings per common share and common share equivalents	$ 0.29		$ 0.27		$ 0.86		$ 0.82

[1] Options to purchase 2,579,585 shares of the Company's common stock that were outstanding in the three and nine months ended September 30, 2012, at a weighted average exercise price of $16.86, were excluded from the respective computations of diluted EPS because their inclusion would have had an antidilutive effect. Options to purchase 8,918,493 and 3,122,124 shares, respectively, of the Company's common stock that were outstanding in the three and nine months ended September 30, 2011, at respective weighted average exercise prices of $15.64 and $17.32, were excluded from the respective computations of diluted EPS because their inclusion would have had an antidilutive effect.