Employee Benefit Plans - Schedule of Weighted-Average Assumptions Used in Calculating Net Periodic Benefit Costs (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Pension Benefits
Assumptions for benefit obligations at measurement date:
Discount rate	3.50%	4.00%	4.20%
Assumptions for periodic benefit cost for the year ended:
Discount rate	4.00%	4.20%	3.90%
Expected long-term rate of return on plan assets	6.50%	6.50%	6.50%
Other Postretirement Benefits
Assumptions for benefit obligations at measurement date:
Discount rate	3.50%	4.00%	4.20%
Assumptions for periodic benefit cost for the year ended:
Discount rate	4.00%	4.20%	3.90%
Expected long-term rate of return on plan assets	6.50%	6.50%	6.50%
Assumptions for year-end valuations:
Health care cost trend rate assumed for next year, pre-age 65	6.50%	6.70%	7.00%
Health care cost trend rate assumed for next year, post-age 65	6.50%	6.80%	7.10%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate)	4.50%	4.50%	4.50%
Year the rate reaches the ultimate trend rate	2037	2037	2037