Borrowings (Summary of Short term and Long term Borrowings and Subordinated Debentures) (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Securities Sold under Agreements to Repurchase	[1]	$ 244,646	$ 230,612
FHLBB borrowings
Debt Instrument [Line Items]
Weighted Average Rate (as a percent)		1.87%	1.87%
Maturities of Long-term Debt and Capital Lease Obligations [Abstract]
2018		$ 250,000
2020		10,000
Customer repurchase agreements
Maturities of Long-term Debt and Capital Lease Obligations [Abstract]
2018		244,646
FHLBB and correspondent bank overnight borrowings
Maturities of Long-term Debt and Capital Lease Obligations [Abstract]
2018		47,150
Wholesale repurchase agreements
Maturities of Long-term Debt and Capital Lease Obligations [Abstract]
2018		$ 0
Capital lease obligation
Debt Instrument [Line Items]
Weighted Average Rate (as a percent)		4.18%	4.31%
Maturities of Long-term Debt and Capital Lease Obligations [Abstract]
2018		$ 71
2019		76
2020		81
2021		90
2022		98
Thereafter		$ 375
Long-term Debt
Debt Instrument [Line Items]
Weighted Average Rate (as a percent)		2.03%	2.05%
Maturities of Long-term Debt and Capital Lease Obligations [Abstract]
2019		$ 76
2020		10,081
2021		90
2022		98
Thereafter		375
Total long-term borrowings		$ 10,720	$ 10,791
Subordinated Debt and Junior Subordinated Debt
Debt Instrument [Line Items]
Weighted Average Rate (as a percent)		5.66%	5.69%
Maturities of Long-term Debt and Capital Lease Obligations [Abstract]
Thereafter		$ 58,911
Total long-term borrowings		$ 58,911	$ 58,755
Subordinated Debt | Subordinated debentures
Debt Instrument [Line Items]
Weighted Average Rate (as a percent)		5.50%	5.50%
Maturities of Long-term Debt and Capital Lease Obligations [Abstract]
Thereafter		$ 14,580
Total long-term borrowings		14,580	$ 14,526
Debt Issuance Costs, Net		420	474
Short-term Debt
Debt Instrument [Line Items]
Total short-term borrowings		$ 541,867	$ 530,129
Weighted Average Rate (as a percent)		1.10%	0.74%
Maturities of Long-term Debt and Capital Lease Obligations [Abstract]
2018		$ 541,867
Short-term Debt | FHLBB borrowings
Debt Instrument [Line Items]
FHLBB borrowings		250,000	$ 210,000
Short-term Debt | Customer repurchase agreements
Debt Instrument [Line Items]
Securities Sold under Agreements to Repurchase		244,646	225,605
Short-term Debt | FHLBB and correspondent bank overnight borrowings
Debt Instrument [Line Items]
FHLBB and correspondent bank overnight borrowings		47,150	89,450
Short-term Debt | Capital lease obligation
Debt Instrument [Line Items]
Capital lease obligation		71	67
Long-term Debt
Maturities of Long-term Debt and Capital Lease Obligations [Abstract]
Long-term Federal Home Loan Bank Advances		10,000
Long-term Debt | FHLBB borrowings
Maturities of Long-term Debt and Capital Lease Obligations [Abstract]
Long-term Federal Home Loan Bank Advances		10,000	10,000
Long-term Debt | Capital lease obligation
Maturities of Long-term Debt and Capital Lease Obligations [Abstract]
Capital lease obligation		720	791
Commercial and Industrial Sector [Member] | Short-term Debt | Wholesale repurchase agreements
Debt Instrument [Line Items]
Securities Sold under Agreements to Repurchase		$ 0	$ 5,007
Subsidiary One [Member] | Junior Subordinated Debt
Debt Instrument [Line Items]
Weighted Average Rate (as a percent)		5.48%	5.50%
Maturities of Long-term Debt and Capital Lease Obligations [Abstract]
Thereafter		$ 36,083
Total long-term borrowings		$ 36,083	$ 36,083
Subsidiary Two [Member] | Junior Subordinated Debt
Debt Instrument [Line Items]
Weighted Average Rate (as a percent)		6.75%	6.85%
Maturities of Long-term Debt and Capital Lease Obligations [Abstract]
Thereafter		$ 8,248
Total long-term borrowings		$ 8,248	$ 8,146

[1] (1) Presented within short-term borrowings on the consolidated statements of condition.