Employee Benefit Plans - Assumptions Used for Postretirement Benefit Obligations and Postretirement Costs (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Retirement Benefits [Abstract]
Postretirement benefit obligations, discount rate	4.00%	4.30%
Postretirement costs, discount rate	4.00%	4.30%
Health care cost trend rate for medical benefits assumed for next year (pre-65)	7.75%	7.50%
Health care cost trend rate for medical benefits assumed for next year (post-65)	6.00%	5.00%
Ultimate trend rate (pre-65)	4.50%	4.50%
Ultimate trend rate (post-65)	4.50%	4.50%
Year that the rate reaches the ultimate trend rate (pre-65)	2030	2023
Year that the rate reaches the ultimate trend rate (post-65)	2023	2018