Loans - Average Recorded Investment and Interest Income Recognized on Impaired Loans (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	$ 155,333	$ 217,263	$ 203,896
Interest Income Recognized	5,010	5,945	5,103
Commercial real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	95,771	131,199	123,393
Interest Income Recognized	3,035	3,198	2,910
Consumer loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	2,214	1,191	943
Interest Income Recognized	49	65	14
Commercial and industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	30,485	55,814	47,940
Interest Income Recognized	1,114	1,686	1,463
Commercial real estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	74,256	110,447	101,972
Interest Income Recognized	2,488	2,946	2,640
Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	21,515	20,752	21,421
Interest Income Recognized	547	252	270
Residential mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	26,863	29,059	31,620
Interest Income Recognized	812	996	716
Home equity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Average Recorded Investment	2,214	1,191	943
Interest Income Recognized	$ 49	$ 65	$ 14