Summary of Significant Accounting Policies - Reconciliation of Numerator and Denominator of Basic Earnings Per Share with that of Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Distributed earnings allocated to common stock									$ 131,527	$ 98,510	$ 89,497
Undistributed earnings allocated to common stock									18,816	48,317	48,218
Net earnings allocated to common shareholders									$ 150,343	$ 146,827	$ 137,715
Average common shares outstanding	104,808,000	104,760,000	99,198,000	80,902,000	76,864,000	76,219,000	71,484,000	69,497,000	97,502,633	73,531,992	69,334,849
Dilutive effect of stock compensation									387,445	361,135	290,682
Average diluted shares outstanding	105,125,000	105,068,000	99,620,000	81,307,000	77,303,000	76,648,000	71,809,000	69,714,000	97,890,078	73,893,127	69,625,531
Earnings per basic common share	$ 0.17	$ 0.54	$ 0.37	$ 0.48	$ 0.51	$ 0.54	$ 0.44	$ 0.50	$ 1.54	$ 2.00	$ 1.99
Earnings per diluted common share	$ 0.17	$ 0.54	$ 0.37	$ 0.48	$ 0.51	$ 0.54	$ 0.44	$ 0.50	$ 1.54	$ 1.99	$ 1.98