Loans, Allowance for Loan Losses and Credit Quality (Internal Risk-Rating Categories for the Commercial Portfolio) (Details) - USD ($)	6 Months Ended
	Jun. 30, 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	4,618,963,000	$ 4,539,256,000
Commercial and Industrial [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	976,264,000	888,528,000
Commercial Real Estate [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	3,131,337,000	3,116,561,000
Commercial Construction [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	364,225,000	401,797,000
Small Business [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	147,137,000	132,370,000
PASS [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	4,443,599,000	4,345,232,000
PASS [Member] | Commercial and Industrial [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	896,554,000	806,331,000
PASS [Member] | Commercial Real Estate [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	3,037,955,000	3,007,672,000
PASS [Member] | Commercial Construction [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	364,225,000	400,964,000
PASS [Member] | Small Business [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	144,865,000	130,265,000
POTENTIAL WEAKNESS [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	66,195,000	87,822,000
POTENTIAL WEAKNESS [Member] | Commercial and Industrial [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	17,294,000	16,563,000
POTENTIAL WEAKNESS [Member] | Commercial Real Estate [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	47,608,000	69,788,000
POTENTIAL WEAKNESS [Member] | Commercial Construction [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	0	0
POTENTIAL WEAKNESS [Member] | Small Business [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	1,293,000	1,471,000
DEFINITE WEAKNESS-LOSS UNLIKELY [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	102,701,000	99,516,000
DEFINITE WEAKNESS-LOSS UNLIKELY [Member] | Commercial and Industrial [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	56,413,000	59,415,000
DEFINITE WEAKNESS-LOSS UNLIKELY [Member] | Commercial Real Estate [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	45,311,000	38,637,000
DEFINITE WEAKNESS-LOSS UNLIKELY [Member] | Commercial Construction [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	0	833,000
DEFINITE WEAKNESS-LOSS UNLIKELY [Member] | Small Business [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	977,000	631,000
PARTIAL LOSS PROBABLE [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	6,468,000	6,686,000
PARTIAL LOSS PROBABLE [Member] | Commercial and Industrial [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	6,003,000	6,219,000
PARTIAL LOSS PROBABLE [Member] | Commercial Real Estate [Member]
Internal risk-rating categories for the Company's commercial portfolio
TOTAL	463,000	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	2,000	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