Employee Benefit Plans - Weighted Average Assumptions Used in Calculating the Net Periodic Benefit Cost (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected long-term rate of return on plan assets	6.60%	6.70%
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	4.22%	4.25%	4.00%
Expected long-term rate of return on plan assets	6.60%	6.70%	6.90%
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate	3.97%	4.00%	3.75%
Expected long-term rate of return on plan assets	6.60%	6.60%	6.90%