Loans, Allowance for Loan Losses and Credit Quality (Internal Risk-Rating Categories for the Commercial Portfolio) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Significant Advanced Considered For Risk Rating Change	$ 50,000
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	4,540,085,000	$ 4,539,256,000
Commercial and Industrial [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	903,214,000	888,528,000
Commercial Real Estate [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	3,102,271,000	3,116,561,000
Commercial Construction [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	400,934,000	401,797,000
Small Business [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	133,666,000	132,370,000
PASS [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	4,352,703,000	4,345,232,000
PASS [Member] | Commercial and Industrial [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	829,401,000	806,331,000
PASS [Member] | Commercial Real Estate [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	2,992,497,000	3,007,672,000
PASS [Member] | Commercial Construction [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	399,568,000	400,964,000
PASS [Member] | Small Business [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	131,237,000	130,265,000
POTENTIAL WEAKNESS [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	89,140,000	87,822,000
POTENTIAL WEAKNESS [Member] | Commercial and Industrial [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	15,910,000	16,563,000
POTENTIAL WEAKNESS [Member] | Commercial Real Estate [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	70,879,000	69,788,000
POTENTIAL WEAKNESS [Member] | Commercial Construction [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	947,000	0
POTENTIAL WEAKNESS [Member] | Small Business [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	1,404,000	1,471,000
DEFINITE WEAKNESS-LOSS UNLIKELY [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	91,654,000	99,516,000
DEFINITE WEAKNESS-LOSS UNLIKELY [Member] | Commercial and Industrial [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	51,782,000	59,415,000
DEFINITE WEAKNESS-LOSS UNLIKELY [Member] | Commercial Real Estate [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	38,431,000	38,637,000
DEFINITE WEAKNESS-LOSS UNLIKELY [Member] | Commercial Construction [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	419,000	833,000
DEFINITE WEAKNESS-LOSS UNLIKELY [Member] | Small Business [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	1,022,000	631,000
PARTIAL LOSS PROBABLE [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	6,588,000	6,686,000
PARTIAL LOSS PROBABLE [Member] | Commercial and Industrial [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	6,121,000	6,219,000
PARTIAL LOSS PROBABLE [Member] | Commercial Real Estate [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	464,000	464,000
PARTIAL LOSS PROBABLE [Member] | Commercial Construction [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	0	0
PARTIAL LOSS PROBABLE [Member] | Small Business [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	3,000	3,000
DEFINITE LOSS [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	0	0
DEFINITE LOSS [Member] | Commercial and Industrial [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	0	0
DEFINITE LOSS [Member] | Commercial Real Estate [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	0	0
DEFINITE LOSS [Member] | Commercial Construction [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	0	0
DEFINITE LOSS [Member] | Small Business [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	$ 0	$ 0