Calculation of Both Basic and Diluted Earnings Per Common 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
Schedule Of Earnings Per Share Basic And Diluted [Line Items]
Net income	$ 39,447		$ 35,357		$ 106,798		$ 108,836
Basic weighted-average number of common shares outstanding	197,437,988	[1]	178,507,769	[1]	197,205,865	[1]	178,333,952	[1]
Plus: Common stock equivalents			613		438		4,358
Diluted weighted-average number of common shares outstanding	197,437,988	[1]	178,508,382	[1]	197,206,303	[1]	178,338,310	[1]
Basic	$ 0.20	[1]	$ 0.20	[1]	$ 0.54	[1]	$ 0.61	[1]
Diluted	$ 0.20	[1]	$ 0.20	[1]	$ 0.54	[1]	$ 0.61	[1]

[1] Share data reflects the five percent common stock dividend issued on May 25, 2012.