Allowance for Loan Losses - Performing and Non Performing Loans (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	$ 166,016		$ 144,489
Residential First Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	135,267	[1]	114,458	[2]
Residential Revolving and Junior Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	25,400	[3]	24,045	[4]
Consumer Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	5,349	[5]	5,986	[6]
Substandard | Residential First Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	2,100		2,600
Substandard | Residential Revolving and Junior Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	219		216
Substandard | Consumer Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	$ 1		$ 9

[1] Residential First Mortgages which have been assigned a risk rating grade of Substandard totaled $2.1 million as of December 31, 2014.

[2] Residential First Mortgages which have been assigned a risk rating grade of Substandard totaled $2.6 million as of December 31, 2013.

[3] Residential Revolving and Junior Mortgages which have been assigned a risk rating grade of Substandard totaled $219 thousand as of December 31, 2014.

[4] Residential Revolving and Junior Mortgages which have been assigned a risk rating grade of Substandard totaled $216 thousand as of December 31, 2013.

[5] Consumer Loans which have been assigned a risk rating grade of Substandard totaled $1 thousand as of December 31, 2014.

[6] Consumer Loans which have been assigned a risk rating grade of Substandard totaled $9 thousand as of December 31, 2013.