Income Taxes - Summary of Deferred Tax Assets and Liabilities from Tax Effects of Temporary Differences (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Allowance for loan losses	$ 44,613	$ 39,292
Discount on purchased loans	31,787	41,012
Net operating loss/tax credit carryforwards	15,367	17,052
Deferred compensation	7,785	8,208
Securities impairments	412	20,371
Pension and other defined benefit plans	6,871	2,190
Net unrealized securities losses		13,295
Other	9,503	3,567
Total	116,338	144,987
Valuation allowance	(15,505)	(15,611)
Total deferred tax assets	100,833	129,376
Loan costs	(1,497)	(931)
Depreciation	(11,533)	(9,819)
Prepaid expenses	(705)	(1,261)
Amortizable intangibles	(13,842)	(11,346)
Lease financing	(7,302)	(6,290)
Debt discharge income deferral	(2,829)	(3,402)
Originated mortgage servicing rights	(2,721)	(2,721)
Other		(1,214)
Total deferred tax liabilities	(40,429)	(36,984)
Net deferred tax assets	$ 60,404	$ 92,392