Income Taxes - Schedule of Significant Components of Net Deferred Tax Assets (Liabilities) (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Deferred Tax Assets
Allowance for loan losses, net of recapture	$ 14,458	$ 14,514
Benefit plan accruals	14,624	21,754
Alternative minimum tax credit	2,545	2,545
Net operating loss carryforwards	30,168	31,765
Federal tax credits	3,116	1,779
Deferred gain on securities	1,844	1,976
Acquired loans	24,857	26,956
Operating lease liabilities	30,451
Lease exit obligation		1,025
Unrealized losses on available-for-sale investment securities		11,853
Unrealized losses on held-to-maturity investment securities	2,391	2,497
Tax credit investments and other partnerships	3,196	3,004
Other real estate owned	144	144
Other, net	2,691	3,167
Total deferred tax assets	130,485	122,979
Deferred Tax Liabilities
Accretion on investment securities	(795)	(595)
Purchase accounting	(17,873)	(18,100)
Loan servicing rights	(6,080)	(6,141)
Premises and equipment	(11,019)	(8,507)
Prepaid expenses	(716)	(681)
Unrealized gains on hedges	(167)	(358)
Other, net	(2,287)	(1,549)
Total deferred tax liabilities	(71,055)	(35,931)
Operating lease right-of-use assets	(29,393)
Unrealized gains on available-for-sale investment securities	(2,725)
Net deferred tax assets	$ 59,430	$ 87,048