Fair Values of Assets and Liabilities - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Random sample percentage of securities for each quarter	100.00%
Credit loss recognized	$ 0	$ 0	$ 0
Total gains or losses included in earnings	(77,000)	770,000
Interest rate swaps transferred into Level 3 from Level 2	3,035,000	20,135,000
Corporate securities transferred from Level 3 to Level 2	0	0
Impaired loans considered to be credit risk of non-collection	100,000
Updated appraisal requirement floor	250,000
Other real estate owned	7,197,000	11,728,000
Financial instrument of credit risk	200,000	100,000
Provision for credit losses	11,196,000	19,227,000	20,544,000
Interest rate swap [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Provision for credit losses	900,000
Loans Held for Sale [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total gains or losses included in earnings	(77,000)	(625,000)
Interest rate swaps transferred into Level 3 from Level 2	3,035,000	20,135,000
Corporate securities transferred from Level 3 to Level 2	0	0
Other Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total gains or losses included in earnings	0	0
Interest rate swaps transferred into Level 3 from Level 2	0	0
Corporate securities transferred from Level 3 to Level 2	$ 0	$ 0