Income Taxes - Schedule of Significant Components of Net Deferred Tax Assets (Liabilities) (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Deferred Tax Assets
Allowance for loan losses, net of recapture	$ 17,838	$ 17,125
Benefit plan accruals	19,418	18,066
AMT credit	17,152	18,378
Unrealized losses on benefit plans	3,964	4,507
Net operating loss carryforwards	66,923	2,041
Premises and equipment	7,301	12,735
Federal tax credits	35	422
Other-than-temporary impairment	3,620	3,558
Acquired loans	54,000	34,870
Lease exit obligation	2,818	2,626
Unrealized losses on available-for-sale investment securities		3,002
Unrealized losses on held-to-maturity investment securities	7,406	7,724
Unrealized losses on hedges	10,532	5,685
Other real estate owned	3,272
Other, net	2,550	4,914
Total deferred tax assets	216,829	135,653
Deferred Tax Liabilities
Accretion on investment securities	(649)	(599)
Other real estate owned		(284)
Purchase accounting	(18,291)	(16,615)
FDIC indemnification asset		(2,565)
Loan servicing rights	(9,674)	(3,890)
Unrealized losses on available-for-sale investment securities	(7,052)
Other, net	(1,715)	(1,716)
Total deferred tax liabilities	(37,381)	(25,669)
Net deferred tax assets	$ 179,448	$ 109,984