Allowance for Loan Losses - Performing and Non Performing Loans (Parenthetical) (Detail) - USD ($) $ in Thousands	Sep. 30, 2016		Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	$ 214,377		$ 195,917
Residential First Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	185,373	[1]	164,405	[2]
Residential First Mortgages | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	3,300		3,900
Residential Revolving and Junior Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	25,389	[3]	26,497	[4]
Residential Revolving and Junior Mortgages | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	265		372
Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	3,615	[5]	5,015	[6]
Consumer and Other Loans | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	$ 0		$ 0

[1] Residential First Mortgages which have been assigned a risk rating grade of Substandard totaled $3.3 million as of September 30, 2016.

[2] Residential First Mortgages which have been assigned a risk rating grade of Substandard totaled $3.9 million as of December 31, 2015.

[3] Residential Revolving and Junior Mortgages which have been assigned a risk rating grade of Substandard totaled $265 thousand as of September 30, 2016.

[4] Residential Revolving and Junior Mortgages which have been assigned a risk rating grade of Substandard totaled $372 thousand as of December 31, 2015.

[5] No Consumer Loans had been assigned a risk rating grade of Substandard as of September 30, 2016.

[6] No Consumer Loans had been assigned a risk rating grade of Substandard as of December 31, 2015.