Employee Benefits (Defined benefit and postretirement benefit plans) (Details 2)	12 Months Ended
	Dec. 31, 2017 USD ($) plan Y	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Pension Benefits
Employee benefits
Number of defined benefit plans | plan	2
Reconciliation of changes in benefit obligations
Benefit Obligation at Beginning of Year	$ 107,593,000	$ 105,993,000
Service Cost	0	0	$ 0
Interest Cost	4,665,000	4,882,000	4,655,000
Defined Benefit Plan, Plan Amendments	0	0
Actuarial Losses (Gains)	6,828,000	2,708,000
Benefit Obligation at End of Year	110,080,000	107,593,000	105,993,000
Reconciliation of changes in fair value of plan assets
Fair Value of Plan Assets at Beginning of Year	83,383,000	83,858,000
Actual Return on Plan Assets	12,047,000	5,031,000
Employer Contributions	10,484,000	484,000
Employer Benefits Paid	(9,006,000)	(5,990,000)
Fair Value of Plan Assets at End of Year	96,908,000	83,383,000	83,858,000
Funded Status at End of Year	(13,172,000)	(24,210,000)
Amounts Recognized in Accumulated Other Comprehensive Income (Loss), Net of Tax
Net Actuarial Gains (Losses)	32,730,000	33,954,000
Prior Service Credit	0	0
Total Amounts Recognized in Accumulated Other Comprehensive Income, Net of Tax	$ (32,730,000)	(33,954,000)
Postretirement Benefits
Employee benefits
Retirees' age and above which Medicare supplemental plan subsidy is provided (in years) | Y	65
Limit on annual credit provided in HRA to eligible employees	$ 1,200
Reconciliation of changes in benefit obligations
Benefit Obligation at Beginning of Year	24,308,000	25,307,000
Service Cost	453,000	513,000	621,000
Interest Cost	1,093,000	1,134,000	1,155,000
Defined Benefit Plan, Plan Amendments	(2,730,000)	0
Actuarial Losses (Gains)	2,052,000	(1,846,000)
Benefit Obligation at End of Year	24,206,000	24,308,000	25,307,000
Reconciliation of changes in fair value of plan assets
Fair Value of Plan Assets at Beginning of Year	0	0
Actual Return on Plan Assets	0	0
Employer Contributions	970,000	800,000
Employer Benefits Paid	(970,000)	(800,000)
Fair Value of Plan Assets at End of Year	0	0	$ 0
Funded Status at End of Year	(24,206,000)	(24,308,000)
Participants contributions	500,000	800,000
Amounts Recognized in Accumulated Other Comprehensive Income (Loss), Net of Tax
Net Actuarial Gains (Losses)	(3,143,000)	(4,647,000)
Prior Service Credit	1,872,000	415,000
Total Amounts Recognized in Accumulated Other Comprehensive Income, Net of Tax	5,015,000	5,062,000
Excess retirement Plan | Pension Benefits
Employee benefits
Accumulated benefit obligation	$ 3,900,000	$ 4,000,000