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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Common Share:
Net income	$ 25,135	$ 27,682	$ 29,520	$ 33,835	$ 39,608	$ 27,121	$ 33,922	$ 31,310	$ 116,172	$ 131,961	$ 143,627
Basic weighted-average number of common shares outstanding	221,471,635	200,614,091	200,472,592	200,128,384	199,613,524	199,445,874	199,244,243	198,924,995	205,716,293	199,309,425	197,354,159
Plus: Common stock equivalents									0	0	213
Diluted weighted-average number of common shares outstanding	221,471,635	200,614,091	200,472,592	200,128,384	199,613,524	199,445,874	199,244,243	198,924,995	205,716,293	199,309,425	197,354,372
Earnings per common share:
Basic (in usd per share)	$ 0.11	$ 0.14	$ 0.15	$ 0.17	$ 0.20	$ 0.14	$ 0.17	$ 0.16	$ 0.56	$ 0.66	$ 0.73
Diluted (in usd per share)	$ 0.11	$ 0.14	$ 0.15	$ 0.17	$ 0.20	$ 0.14	$ 0.17	$ 0.16	$ 0.56	$ 0.66	$ 0.73