Pension and Other Postretirement Benefit Costs - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1,881	$ 1,978	$ 5,621	$ 5,866
Interest cost	4,484	4,607	13,428	13,755
Expected return on plan assets	(5,112)	(5,017)	(15,337)	(15,051)
Defined Benefit Plan, Amortization of Gains (Losses)	3,656	3,555	10,899	10,559
Amortization of net actuarial loss	32	57	95	173
Settlement expense	0	193		193
Net periodic benefit cost	4,941	5,373	14,706	15,495
Amount allocated to construction	(1,581)	(1,556)	(4,660)	(4,678)
Amount deferred to regulatory balancing account(1)	(1,484)	(1,542)	(4,519)	(4,762)
Net amount charged to expense	1,876	2,275	5,527	6,055
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	98	119	295	361
Interest cost	274	301	822	901
Expected return on plan assets	0	0	0	0
Defined Benefit Plan, Amortization of Gains (Losses)	138	192	415	575
Amortization of net actuarial loss	(117)	(117)	(351)	(351)
Settlement expense	0	0
Net periodic benefit cost	393	495	1,181	1,486
Amount allocated to construction	(136)	(163)	(403)	(491)
Amount deferred to regulatory balancing account(1)	0	0	0	0
Net amount charged to expense	$ 257	$ 332	778	995
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Settlement expense			0	$ 0
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Settlement expense			$ 0