Employee Benefit Plans - Assumptions in Benefit Plans (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted-average assumptions used to determine projected pension benefit obligations
Discount rate (as a percent)	3.85%	4.30%	4.25%
Health care cost trend rate for medical benefits assumed for next year (pre-65)	7.50%	7.50%	5.50%
Health care cost trend rate for medical benefits assumed for next year (post-65)	5.75%	5.00%	5.50%
Ultimate trend rate (pre-65)	4.50%	4.50%	4.50%
Ultimate trend rate (post-65)	4.50%	4.50%	4.50%
Year that the rate reaches the ultimate trend rate (pre-65)	2030	2023	2018
Year that the rate reaches the ultimate trend rate (post-65)	2023	2018	2018
Beginning Discount Rate	3.85%	4.25%	4.00%