Significant Accounting Policies and Basis of Presentation - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2014 CompensationPlan Segment	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2014
Derivative [Line Items]
Average reserve balances maintained with federal reserve bank		$ 61,200,000	$ 50,100,000
Deferred cost of bank loans		0
Contractual payment of principal duration		90 days
Contractual terms for loans		6 months
FDIC Indemnification Asset		42,181,000	66,725,000
Maximum loan to value ratio		80.00%
Delinquency period for loan charge-offs		120 days
Goodwill		3,664,000	3,676,000
FHLB, Federal Reserve Bank, and other restricted stock		82,002,000	42,424,000
Federal Home Loan Bank Stock		71,600,000	32,100,000
Percentage of tax position will realized or sustained upon examination		50.00%
Number of share-based compensation plans		4
Number of reportable Segment		1
Change in estimates recognized in other non-interest income	4,500,000
Increase in recorded interest income	4,500,000
Additional provision for loan losses	7,500,000
Increase in net income net of tax				900,000
Increase in basic earnings per share		$ 0.06
Increase in diluted earnings per share			$ 0.06
Interest Rate Swap | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Notional Amount		251,900,000	150,300,000
Interest Rate Swap | Cash Flow Hedging | Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Notional Amount		150,000,000			150,000,000
Credit Risk Contract | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Notional Amount		13,400,000			13,400,000
ESPP
Derivative [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price of Common Stock, Percent		85.00%
Share-based Compensation Arrangement by Share-based Payment Award, Discount from Market Price, Purchase Date		15.00%
Federal Deposit Insurance Corporation
Repurchase Agreement Counterparty [Line Items]
Other Real Estate, Covered		$ 9,400,000	$ 7,000,000