Postretirement Benefits (Details 2)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee pension plan | Qualified plan | Funded plan
Actuarial assumptions used to determine benefit obligations:
Discount rate (as a percent)	3.73%	4.24%	4.57%	4.17%
Compensation increases (as a percent)	3.32%	3.32%	3.32%	3.32%
Actuarial assumptions used to determine net periodic benefit cost:
Discount rate (as a percent)	4.24%	4.57%	4.17%
Expected return on assets (as a percent)	6.75%	7.00%	7.00%	7.00%
Rate of compensation increase (as a percent)	3.32%	3.32%	3.32%
Supplemental employee retirement plan (SERP) | Nonqualified plan | Unfunded plan
Actuarial assumptions used to determine benefit obligations:
Pre-retirement discount rate (as a percent)	3.73%	4.24%	4.57%	4.17%
Post-retirement discount rate (as a percent)	3.23%	3.74%	4.07%	3.67%
Compensation increases (as a percent)	5.00%	5.00%	5.00%	5.00%
Actuarial assumptions used to determine net periodic benefit cost:
Pre-retirement discount rate (as a percent)	4.24%	4.57%	4.17%
Post-retirement discount rate (as a percent)	3.74%	4.07%	3.67%
Rate of compensation increase (as a percent)	5.00%	5.00%	5.00%