Summary of Significant Accounting Policies - Calculation of Both Basic and Diluted Earnings Per Common Share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Net income	$ 36,829	$ 39,447	$ 32,820	$ 34,531	$ 24,532	[1]	$ 35,072	[1]	$ 36,608	[1]	$ 36,299	[1]	$ 143,627	$ 132,511	$ 130,006
Basic weighted-average number of common shares outstanding	197,795,817	197,437,988	197,246,322	196,930,733	178,694,711		178,507,769		178,335,522		178,154,684		197,354,159	178,424,883	177,568,546
Plus: Common stock equivalents													213	1,187	9,117
Diluted weighted-average number of common shares outstanding	197,795,817	197,437,988	197,250,168	196,961,915	178,695,174		178,508,382		178,345,558		178,162,788		197,354,372	178,426,070	177,577,663
Basic	$ 0.19	$ 0.20	$ 0.17	$ 0.18	$ 0.14		$ 0.20		$ 0.21		$ 0.20		$ 0.73	$ 0.74	$ 0.73
Diluted	$ 0.19	$ 0.20	$ 0.17	$ 0.18	$ 0.14		$ 0.20		$ 0.21		$ 0.20		$ 0.73	$ 0.74	$ 0.73

[1] Previously reported amounts have been revised to reflect an increase in non-interest expense due to the immaterial correction of an error. See the "Correction of an Immaterial Error" section of Note 1 to the consolidated financial statements for additional information.