Pension and Other Postretirement Benefit Costs - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1,807	$ 1,870	$ 3,614	$ 3,740
Interest cost	4,183	4,472	8,366	8,944
Expected return on plan assets	(5,150)	(5,112)	(10,301)	(10,225)
Amortization of net actuarial loss	10	31	21	63
Defined Benefit Plan, Amortization of Gain (Loss)	4,524	3,622	9,047	7,243
Net periodic benefit cost	5,374	4,883	10,747	9,765
Amount allocated to construction	(685)	(1,558)	(1,367)	(3,079)
Amount deferred to regulatory balancing account(1)	(2,747)	(1,508)	(5,503)	(3,035)
Net amount charged to expense	1,942	1,817	3,877	3,651
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	79	99	159	197
Interest cost	241	274	482	548
Expected return on plan assets	0	0	0	0
Amortization of net actuarial loss	(117)	(117)	(234)	(234)
Defined Benefit Plan, Amortization of Gain (Loss)	112	139	222	277
Net periodic benefit cost	315	395	629	788
Amount allocated to construction	(28)	(135)	(55)	(267)
Amount deferred to regulatory balancing account(1)	0	0	0	0
Net amount charged to expense	$ 287	$ 260	$ 574	$ 521