Allowance for Loan Losses and Credit Quality of Loans, Changes in Allowance for Loan Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Changes in allowance for loan losses by portfolio segment [Roll Forward]
Balance, beginning of period	$ 65,200	$ 63,018	$ 66,359
Charge-offs	(33,087)	(29,299)	(26,087)
Recoveries	6,399	6,050	4,461
Provision	30,988	25,431	18,285
Balance, end of period	69,500	65,200	63,018
Commercial Loans [Member]
Changes in allowance for loan losses by portfolio segment [Roll Forward]
Balance, beginning of period	25,444	25,545	32,433
Charge-offs	(4,169)	(4,592)	(5,718)
Recoveries	1,077	1,887	1,014
Provision	5,254	2,604	(2,184)
Balance, end of period	27,606	25,444	25,545
Consumer Loans [Member]
Changes in allowance for loan losses by portfolio segment [Roll Forward]
Balance, beginning of period	33,375	29,253	26,720
Charge-offs	(27,072)	(23,364)	(18,140)
Recoveries	5,142	3,870	3,127
Provision	25,385	23,616	17,546
Balance, end of period	36,830	33,375	29,253
Residential Real Estate Mortgages [Member]
Changes in allowance for loan losses by portfolio segment [Roll Forward]
Balance, beginning of period	6,381	7,960	7,130
Charge-offs	(1,846)	(1,343)	(2,229)
Recoveries	180	293	320
Provision	349	(529)	2,739
Balance, end of period	5,064	6,381	7,960
Unallocated [Member]
Changes in allowance for loan losses by portfolio segment [Roll Forward]
Balance, beginning of period	0	260	76
Charge-offs	0	0	0
Recoveries	0	0	0
Provision	0	(260)	184
Balance, end of period	0	0	260
Acquired Loans [Member]
Changes in allowance for loan losses by portfolio segment [Roll Forward]
Balance, beginning of period	700
Charge-offs	(700)	(500)	$ (2,700)
Balance, end of period	$ 0	$ 700