Income Taxes - Schedule of Significant Components of Net Deferred Tax Assets (Liabilities) (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Deferred Tax Assets
Allowance for loan losses, net of recapture	$ 14,651	$ 14,514
Benefit plan accruals	17,144	21,754
Alternative minimum tax credit	1,272	2,545
Net operating loss carryforwards	25,369	31,765
Federal tax credits		1,779
Deferred gain on securities	1,563	1,976
Acquired loans	18,872	26,956
Operating lease liabilities	28,460	0
Lease exit obligation		1,025
Unrealized losses on available-for-sale investment securities		11,853
Unrealized losses on held-to-maturity investment securities	1,869	2,497
Tax credit investments and other partnerships	1,501	3,004
Other real estate owned	156	144
Other, net	738	3,167
Total deferred tax assets	111,595	122,979
Deferred Tax Liabilities
Accretion on investment securities	(897)	(595)
Purchase accounting	(17,575)	(18,100)
Loan servicing rights	(6,104)	(6,141)
Premises and equipment	(11,840)	(8,507)
Prepaid expenses	(673)	(681)
Operating lease right-of-use assets	(27,348)	0
Unrealized gains on available-for-sale investment securities	(18,152)	0
Unrealized gains on hedges	(374)	(358)
Other, net	(2,109)	(1,549)
Total deferred tax liabilities	(85,072)	(35,931)
Net deferred tax assets	$ 26,523	$ 87,048