Allowance for Loan Losses - Performing and Non Performing Loans (Parenthetical) (Detail) - USD ($) $ in Thousands	Mar. 31, 2017		Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	$ 235,520		$ 223,976
Residential First Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	205,106	[1]	194,007	[2]
Residential First Mortgages | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	3,100		3,300
Residential Revolving and Junior Mortgages
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	27,090	[3]	26,425	[4]
Residential Revolving and Junior Mortgages | Substandard
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	1,900		1,100
Consumer and Other Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Risk rating grade totaled	3,324	[5]	3,544	[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 $3.1million as of March 31, 2017.

[2] Residential First Mortgages which have been assigned a risk rating grade of Substandard totaled $3.3 million as of December 31, 2016.

[3] Residential Revolving and Junior Mortgages which have been assigned a risk rating grade of Substandard totaled $1.9 million as of March 31, 2017.

[4] Residential Revolving and Junior Mortgages which have been assigned a risk rating grade of Substandard totaled $1.1 million as of December 31, 2016.

[5] No Consumer Loans had been assigned a risk rating grade of Substandard as of March 31, 2017.

[6] No Consumer Loans had been assigned a risk rating grade of Substandard as of December 31, 2016.