Employee Benefit Plans - Assumptions Used to Determine Benefit Obligation and Net Periodic Cost (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure
Weighted average discount rate (percent)	3.69%	4.20%
Weighted average expected long-term rate of return on plan assets (percent)	8.96%	8.94%	8.94%
Assumed healthcare cost and trend rate (percent)	7.00%	6.50%	7.00%
Pension Benefits
Defined Benefit Plan Disclosure
Weighted average discount rate (percent)	4.14%	4.57%	4.13%
Other Postretirement Benefits
Defined Benefit Plan Disclosure
Weighted average discount rate (percent)	4.19%	4.53%	4.13%
Other Postemployment Benefits
Defined Benefit Plan Disclosure
Weighted average discount rate (percent)	4.14%	4.50%	4.13%