Employee Benefits, Weighted-Average Assumptions Used to Determine Benefit Obligations (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate (percent)	3.25%	3.60%
Expected return on plan assets (percent)	7.25%	7.25%	7.50%
Rate of compensation increase (percent)	3.86%	3.86%
SRP
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate (percent)	3.40%	3.85%
Expected return on plan assets (percent)	7.25%	7.25%	7.50%
Rate of compensation increase (percent)	3.86%	3.86%
PBOP
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate (percent)	3.70%	4.20%
Expected return on plan assets (percent)	5.30%	5.30%	5.30%