Loans - Summary of Impaired Financial Receivables (Summary of Purchased Non-Covered Impaired Loans) (Detail) - USD ($)	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings not included above	$ 14,000,000	$ 15,300,000	$ 21,100,000	$ 14,000,000	$ 21,100,000	$ 15,300,000
Purchased Non-Covered Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income recognized on impaired loans	143,000	64,000	41,000	161,000	41,000	132,000
Foregone interest income on impaired loans	451,000	521,000	426,000	923,000	652,000	1,759,000
Nonaccrual loans	17,444,000	18,249,000	15,770,000	17,444,000	15,770,000	18,249,000
Troubled debt restructurings not included above	6,792,000	1,212,000	0	6,792,000	0	1,212,000
Total impaired loans	$ 24,236,000	$ 19,461,000	$ 15,770,000	$ 24,236,000	$ 15,770,000	$ 19,461,000