Goodwill and Other Intangible Assets - Summary of Core Deposit Intangibles (CDI) (Detail) - Core Deposit Intangibles [Member] $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Finite-Lived Intangible Assets [Line Items]
Core deposit intangibles, Beginning balance	$ 9,558
Core deposit intangibles, Reductions/Amortization	0
Core deposit intangibles, Additions	27,605
Core deposit intangibles, Ending balance	37,163
Accumulated amortization, Beginning balance	(4,384)
Accumulated amortization, Reductions/Amortization	(2,068)
Accumulated amortization, Additions	0
Accumulated amortization, Ending balance	(6,452)
Core deposit intangibles, net, Beginning balance	5,174
Core deposit intangibles, net, Reductions/Amortization	(2,068)
Core deposit intangibles, net, Additions	27,605
Core deposit intangibles, net, Ending balance	$ 30,711