Postretirement Benefits (Details 2)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Employee pension plan
Actuarial assumptions used to determine benefit obligations:
Discount rate (as a percent)		4.24%	4.57%	4.17%	5.11%
Compensation increases (as a percent)		3.32%	3.32%	3.32%	4.15%
Actuarial assumptions used to determine net periodic benefit cost:
Discount rate (as a percent)		4.57%	4.17%	5.11%
Expected return on assets (as a percent)		7.00%	7.00%	7.00%	7.50%
Rate of compensation increase (as a percent)		3.32%	3.32%	4.15%
Employee pension plan | Expected return on assets for 2017
Actuarial assumptions used to determine net periodic benefit cost:
Expected return on assets (as a percent)	6.75%
Supplemental employee retirement plan (SERP)
Actuarial assumptions used to determine benefit obligations:
Pre-retirement discount rate (as a percent)		4.24%	4.57%	4.17%	5.11%
Post-retirement discount rate (as a percent)		3.74%	4.07%	3.67%	4.61%
Compensation increases (as a percent)		5.00%	5.00%	5.00%	6.00%
Actuarial assumptions used to determine net periodic benefit cost:
Pre-retirement discount rate (as a percent)		4.57%	4.17%	5.11%
Post-retirement discount rate (as a percent)		4.07%	3.67%	4.61%
Rate of compensation increase (as a percent)		5.00%	5.00%	6.00%