Goodwill and Other Intangible Assets - Summary of Core Deposit Intangibles, Customer and Renewal Lists And Other Intangible Assets (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 131,254	$ 119,048
Accumulated amortization	(83,750)	(71,440)
Core deposit intangibles, Total	47,504	47,608
Core Deposit Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	108,593	97,698
Accumulated amortization	(71,818)	(62,793)
Core deposit intangibles, Total	36,775	34,905
Customer And Renewal Lists [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	10,970	10,970
Accumulated amortization	(7,099)	(6,407)
Core deposit intangibles, Total	3,871	4,563
Other Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	11,691	10,380
Accumulated amortization	(4,833)	(2,240)
Core deposit intangibles, Total	$ 6,858	$ 8,140