Pension and Other Postretirement Benefit Costs - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1,870	$ 1,944	$ 3,740	$ 3,888
Interest cost	4,472	4,574	8,944	9,148
Expected return on plan assets	(5,112)	(5,017)	(10,225)	(10,034)
Amortization of prior service costs	31	58	63	116
Amortization of net actuarial loss	3,622	3,502	7,243	7,004
Net periodic benefit cost	4,883	5,061	9,765	10,122
Amount allocated to construction	(1,558)	(1,574)	(3,079)	(3,122)
Amount deferred to regulatory balancing account(1)	(1,508)	(1,593)	(3,035)	(3,220)
Net amount charged to expense	1,817	1,894	3,651	3,780
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	99	121	197	242
Interest cost	274	300	548	600
Expected return on plan assets	0	0	0	0
Amortization of prior service costs	(117)	(117)	(234)	(234)
Amortization of net actuarial loss	139	192	277	384
Net periodic benefit cost	395	496	788	992
Amount allocated to construction	(135)	(164)	(267)	(328)
Amount deferred to regulatory balancing account(1)	0	0	0	0
Net amount charged to expense	$ 260	$ 332	$ 521	$ 664