Goodwill and Other Intangible Assets - Schedule of Gross Carrying Amount and Accumulated Amortization of Other Intangible Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 102,623	$ 111,850
Accumulated Amortization and Impairment	(61,705)	(76,542)
Net Carrying Amount	40,918	35,308	$ 38,124
Core Deposit [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	81,663	60,103
Accumulated Amortization and Impairment	(50,553)	(43,982)
Net Carrying Amount	31,110	16,121
Customer Business Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount		30,787
Accumulated Amortization and Impairment		(23,341)
Net Carrying Amount		7,446
Customer Trust Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	16,547	16,547
Accumulated Amortization and Impairment	(7,274)	(5,286)
Net Carrying Amount	9,273	11,261
Customer Loan Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	4,413	4,413
Accumulated Amortization and Impairment	(3,878)	(3,933)
Net Carrying Amount	$ 535	$ 480