Earnings Per Share - 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		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Distributed earnings allocated to common stock	$ 35,516	$ 34,562	$ 71,195	$ 61,284
Undistributed earnings allocated to common stock	30,637	2,440	56,557	14,454
Net earnings allocated to common shareholders	$ 66,153	$ 37,002	$ 127,752	$ 75,738
Average common shares outstanding	104,682,910	99,197,807	104,770,681	90,100,627
Equivalents from stock options	269,878	422,238	287,333	469,662
Average diluted shares outstanding	104,952,788	99,620,045	105,058,014	90,570,289
Earnings per basic common share	$ 0.63	$ 0.37	$ 1.22	$ 0.84
Earnings per diluted common share	$ 0.63	$ 0.37	$ 1.22	$ 0.84