Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block]: Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value (Details) $ in Thousands	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Available For Sale Securities Continuous Unrealized Loss Position Less Than Twelve Months Fair Value	$ 251,635	$ 331,210
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,949	9,284
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	135,465	48,785
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	4,406	2,980
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	387,100	379,995
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 7,355	$ 12,264
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	61	156
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	76	23
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	137	179
Corporate Debt Securities
Available For Sale Securities Continuous Unrealized Loss Position Less Than Twelve Months Fair Value	$ 85,642	$ 145,205
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,250	3,818
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	44,640	19,841
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1,597	1,672
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	130,282	165,046
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2,847	5,490
Residential Mortgage Backed Securities Member
Available For Sale Securities Continuous Unrealized Loss Position Less Than Twelve Months Fair Value	1,381	5,038
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	45	116
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	5,237	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	135	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	6,618	5,038
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	180	116
US Treasury Securities
Available For Sale Securities Continuous Unrealized Loss Position Less Than Twelve Months Fair Value	75,811	28,406
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	198	441
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	9,302	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	198	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	85,113	28,406
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	396	441
US Government-sponsored Enterprises Debt Securities
Available For Sale Securities Continuous Unrealized Loss Position Less Than Twelve Months Fair Value	0	3,640
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	166
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	9,669	6,220
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	205	256
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	9,669	9,860
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	205	422
US States and Political Subdivisions Debt Securities
Available For Sale Securities Continuous Unrealized Loss Position Less Than Twelve Months Fair Value	83,682	144,357
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,348	4,561
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	66,617	18,132
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	2,271	554
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	150,299	162,489
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	3,619	5,115
Foreign Government Debt
Available For Sale Securities Continuous Unrealized Loss Position Less Than Twelve Months Fair Value	2,959	3,738
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	90	157
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,959	3,738
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	90	157
Debt Securities
Available For Sale Securities Continuous Unrealized Loss Position Less Than Twelve Months Fair Value	249,475	330,384
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,931	9,259
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	135,465	47,508
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	4,406	2,930
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	384,940	377,892
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	7,337	12,189
Nonredeemable Preferred Stock
Available For Sale Securities Continuous Unrealized Loss Position Less Than Twelve Months Fair Value	2,160	826
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	18	25
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	1,277
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	50
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,160	2,103
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	18	75
Equity Securities
Available For Sale Securities Continuous Unrealized Loss Position Less Than Twelve Months Fair Value	2,160	826
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	18	25
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	1,277
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	50
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,160	2,103
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 18	75
Redeemable Preferred Stock
Available For Sale Securities Continuous Unrealized Loss Position Less Than Twelve Months Fair Value		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		3,315
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		448
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		3,315
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		$ 448