Property, Plant and Equipment (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	$ 3,298,856	$ 3,204,635	$ 3,098,112
Less: Accumulated depreciation	(984,998)	(960,477)	(942,558)
Total property, plant, and equipment, net	2,313,858	2,244,158	2,155,554
Capital expenditures in accrued liabilities (2)	22,112	34,761	42,574
Other non-current liabilities	140,163	135,292	134,793
Utility Plant
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	3,035,089	2,975,217	2,901,791
Work in progress	192,496	159,924	127,383
Less: Accumulated depreciation	(966,766)	(942,879)	(925,589)
Total property, plant, and equipment, net	2,260,819	2,192,262	2,103,585
Non Utility Plant
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	65,743	65,372	63,964
Work in progress	5,528	4,122	4,974
Less: Accumulated depreciation	(18,232)	(17,598)	(16,969)
Total property, plant, and equipment, net	$ 53,039	51,896	$ 51,969
New Headquarters Lease [Member]
Property, Plant and Equipment [Line Items]
Lessee, Operating Lease, Term of Contract	20 years
Lease Expected Commencement Year	2020
New Headquarters Lease [Member] | Build to Suit Lease [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	$ 7,600	500
Other non-current liabilities	$ 7,600	$ 500
Current Headquarters Lease [Member]
Property, Plant and Equipment [Line Items]
Lease Expiration Date	Jan. 01, 2020