Loans - Schedule of Purchased Credit Impaired Loans for which Payments Would Not be Collected (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Financing Receivable Impaired [Line Items]
Total loans	$ 2,163,897	$ 2,016,141
Purchased credit impairment loans, Net carrying amount	2,148,942	1,999,877
Commercial Loans [Member]
Financing Receivable Impaired [Line Items]
Total loans	931,767	822,616
Residential Mortgage Loans [Member]
Financing Receivable Impaired [Line Items]
Total loans	949,022	920,031
Consumer Loans [Member]
Financing Receivable Impaired [Line Items]
Total loans	283,108	273,494
Purchased Credit Impaired Loans [Member]
Financing Receivable Impaired [Line Items]
Total loans	1,083	1,194	$ 1,204
Purchased credit impairment loans, Net carrying amount	1,083	1,139
Purchased Credit Impaired Loans [Member] | Commercial Loans [Member]
Financing Receivable Impaired [Line Items]
Total loans	$ 1,083	$ 1,194