Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Narrative) (Loans) (Detail)	12 Months Ended
	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Threshold Period Past Due for Nonperforming Status of Financing Receivables	90 days
Threshold period past due for write-off of financing receivable (in days)	180 days
Residential Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Threshold Period Past Due for Nonperforming Status of Financing Receivables	90 days
Commercial Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Threshold Period Past Due for Nonperforming Status of Financing Receivables	90 days
Other Real Estate Owned [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Threshold period past due for write-off of financing receivable (in days)	90 days