Benefit Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Salaries, Wages and Officers' Compensation [Abstract]
Stock-based compensation expense	$ 1,900	$ 1,400	$ 1,500
Defined Benefit Plan Disclosure [Line Items]
Pension cost	5,200	2,300	4,400
Regulatory liabilities	246,504	30,717
Defined contribution plan matching contributions expense	2,100	2,200	2,200
Retiree health care and life insurance benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory liabilities	3,500	3,800	$ 3,400
Other Postretirement Benefit Costs
Defined Benefit Plan Disclosure [Line Items]
Regulatory liabilities	34,100	$ 30,600
Pension Plan [Member] | Directly charged [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	3,000
Pension Plan [Member] | Allocated corporate expenses [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement	$ 1,100