Summary of Significant Accounting Policies - Calculation of Both Basic and Diluted Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Common Share:
Net income available to common shareholders	$ 2,875	$ 33,937	$ 31,991	$ 30,341					$ 99,144	$ 116,172	$ 131,961
Basic weighted-average number of common shares outstanding (in shares)	239,916,562	232,737,953	232,565,404	232,338,775	221,471,635	200,614,091	200,472,592	200,128,384	234,405,909	205,716,293	199,309,425
Plus: Common stock equivalents (in shares)									31,091	0	0
Diluted weighted-average number of common shares outstanding (in shares)	239,972,546	232,780,219	232,586,616	232,341,921	221,471,635	200,614,091	200,472,592	200,128,384	234,437,000	205,716,293	199,309,425
Earnings per common share:
Basic (in usd per share)	$ 0.01	$ 0.15	$ 0.14	$ 0.13	$ 0.11	$ 0.14	$ 0.15	$ 0.17	$ 0.42	$ 0.56	$ 0.66
Diluted (in usd per share)	$ 0.01	$ 0.15	$ 0.14	$ 0.13	$ 0.11	$ 0.14	$ 0.15	$ 0.17	$ 0.42	$ 0.56	$ 0.66