Allowance for Loan Losses - Performing and Non Performing Loans (Parenthetical) (Detail) - USD ($) $ in Thousands	Sep. 30, 2015		Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	$ 187,490		$ 166,016
Residential First Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	156,004	[1]	135,267	[2]
Residential Revolving and Junior Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	26,579	[3]	25,400	[4]
Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	4,907	[5]	5,349	[6]
Substandard | Residential First Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	2,200		2,100
Substandard | Residential Revolving and Junior Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	15		219
Substandard | Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	$ 4		$ 1

[1] Residential First Mortgages which have been assigned a risk rating grade of Substandard totaled $2.2 million as of September 30, 2015.

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

[3] Residential Revolving and Junior Mortgages which have been assigned a risk rating grade of Substandard totaled $15 thousand as of September 30, 2015.

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

[5] Consumer Loans which have been assigned a risk rating grade of Substandard were $4 thousand as of September 30, 2015.

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