Income Taxes - Schedule of Significant Components of Net Deferred Tax Assets (Liabilities) (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred Tax Assets
Allowance for loan losses, net of recapture	$ 19,773	$ 17,125
Benefit plan accruals	23,846	18,066
Alternative minimum tax credit	19,523	18,378
Unrealized losses on benefit plans	205	4,507
Net operating loss carryforwards	66,917	2,041
Premises and equipment		12,735
Federal tax credits	35	422
Other-than-temporary impairment	3,606	3,558
Acquired loans	40,522	34,870
Lease exit obligation	2,060	2,626
Unrealized losses on available-for-sale investment securities	23,365	3,002
Unrealized losses on held-to-maturity investment securities	7,118	7,724
Unrealized losses on hedges	4,116	5,685
Other real estate owned	3,310
Other, net	2,675	4,914
Total deferred tax assets	217,071	135,653
Deferred Tax Liabilities
Accretion on investment securities	(700)	(599)
Other real estate owned		(284)
Purchase accounting	(17,552)	(16,615)
FDIC indemnification asset		(2,565)
Loan servicing rights	(9,627)	(3,890)
Premises and equipment	(4,800)
Other, net	(2,529)	(1,716)
Total deferred tax liabilities	(35,208)	(25,669)
Net deferred tax assets	$ 181,863	$ 109,984