Business Combinations - Summary of Estimated Fair Values of Assets Acquired and Liabilities Assumed at Date of Acquisition (Detail) - USD ($) $ in Thousands	Jan. 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Assets acquired:
Bank owned life insurance		$ 221,190	$ 134,847
Liabilities assumed:
Goodwill recorded in the merger		$ 338,104	$ 150,501
Sun Bancorp, Inc.
Business Acquisition [Line Items]
Total Purchase Price:	$ 474,930
Assets acquired:
Cash and cash equivalents	68,632
Securities	254,522
Loans	1,517,172
Accrued interest receivable	5,621
Bank owned life insurance	85,238
Deferred tax asset	57,712
Other assets	42,105
Total assets acquired	2,042,899
Liabilities assumed:
Deposits	(1,616,073)
Borrowings	(127,747)
Other liabilities	(11,752)
Total liabilities assumed	(1,755,572)
Net assets acquired	287,327
Goodwill recorded in the merger	$ 187,603