Loans (Credit Quality Indicators - Residential, Consumer) (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Current loans	$ 3,353,996,000	$ 3,209,949,000
Past due loans	20,075,000	24,422,000
Accruing Residential Mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Current loans	1,070,501,000	954,867,000
Past due loans	6,413,000	11,017,000
Nonaccrual Residential Mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Current loans	128,102,000	127,731,000
Past due loans	1,442,000	1,209,000
Homeowner Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Current loans	20,790,000	27,924,000
Past due loans	0	0
Residential Real Estate [Member]
Financing Receivable, Recorded Investment [Line Items]
Current loans	1,219,393,000	1,110,522,000
Past due loans	7,855,000	12,226,000
Home Equity Lines [Member]
Financing Receivable, Recorded Investment [Line Items]
Current loans	256,774,000	263,519,000
Past due loans	1,340,000	681,000
Home Equity Loans [Member]
Financing Receivable, Recorded Investment [Line Items]
Current loans	32,552,000	35,762,000
Past due loans	1,801,000	1,510,000
Other Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Current loans	31,484,000	38,342,000
Past due loans	43,000	143,000
Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Current loans	320,810,000	337,623,000
Past due loans	$ 3,184,000	$ 2,334,000