Pension and Other Postretirement Benefit Costs - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Nov. 06, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		$ 1,757	$ 1,881	$ 5,371	$ 5,621
Interest cost		4,336	4,484	12,702	13,428
Expected return on plan assets		(5,143)	(5,112)	(15,444)	(15,337)
Amortization of net actuarial loss		11	32	32	95
Defined Benefit Plan, Amortization of Gain (Loss)		5,650	3,656	14,697	10,899
Net periodic benefit cost		6,611	4,941	17,358	14,706
Amount allocated to construction		(659)	(1,581)	(2,026)	(4,660)
Amount deferred to regulatory balancing account(1)		(3,878)	(1,484)	(9,381)	(4,519)
Net amount charged to expense		2,074	1,876	5,951	5,527
Other Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		80	98	239	295
Interest cost		241	274	723	822
Expected return on plan assets		0	0	0	0
Amortization of net actuarial loss		(117)	(117)	(351)	(351)
Defined Benefit Plan, Amortization of Gain (Loss)		110	138	332	415
Net periodic benefit cost		314	393	943	1,181
Amount allocated to construction		(27)	(136)	(82)	(403)
Amount deferred to regulatory balancing account(1)		0	0	0	0
Net amount charged to expense		$ 287	$ 257	$ 861	$ 778
Subsequent Event [Member] | Scenario, Forecast [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Other Cost (Credit)	$ 8,100