Pension and Other Postretirement Benefits - Weighted average assumptions (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Weighted average assumptions used to determine net cost for years ended December 31
Expected long-term rate of return on plan assets	5.77%
Pension Benefits
Weighted average assumptions used to determine benefit obligations at December 31
Discount rate	4.17%	4.16%
Rate of compensation increase	4.02%	4.07%
Weighted average assumptions used to determine net cost for years ended December 31
Discount rate	4.16%	3.92%	4.73%
Expected long-term rate of return on plan assets	6.23%	6.15%	6.72%
Rate of compensation increase	4.07%	4.05%	4.05%
Postretirement Benefits
Weighted average assumptions used to determine benefit obligations at December 31
Discount rate	4.08%	4.30%
Weighted average assumptions used to determine net cost for years ended December 31
Discount rate	4.30%	3.90%	4.80%