Income Taxes - Schedule of Income Tax Reconciliation (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Income before provision for income taxes									$ 85,502	$ 65,325	$ 35,199
Applicable statutory Federal income tax rate									21.00%	35.00%	35.00%
Computed "expected" Federal income tax expense									$ 17,955	$ 22,864	$ 12,320
(Decrease) increase in Federal income tax expense resulting from
Tax exempt interest									(615)	(839)	(390)
ESOP fair market value adjustment									125	223	131
ESOP dividends									(136)	(230)	(223)
Earnings on BOLI									(1,072)	(1,155)	(781)
Merger related expenses									322	478	1,005
State income taxes net of Federal benefit									85	110	62
Stock compensation									(758)	(1,823)	0
Impact of Tax Cuts and Jobs Act (“Tax Reform”)									(1,854)	3,643	0
Reclassification of certain tax effect from accumulated other comprehensive income									(586)	0	0
Other items, net									104	(416)	29
Income tax expense (benefit)	$ 4,269	$ 5,278	$ 3,018	$ 1,005	$ 10,186	$ 5,700	$ 3,170	$ 3,799	$ 13,570	$ 22,855	$ 12,153