Property, Plant and Equipment (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	$ 3,370,388	$ 3,204,635	$ 3,148,545
Less: Accumulated depreciation	996,994	960,477	954,782
Total property, plant, and equipment, net	2,373,394	2,244,158	2,193,763
Capital expenditures in accrued liabilities (2)	27,692	34,761	41,675
Other non-current liabilities	140,475	135,292	134,123
Utility Plant
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	3,068,234	2,975,217	2,934,424
Work in progress	227,200	159,924	145,148
Less: Accumulated depreciation	978,446	942,879	937,498
Total property, plant, and equipment, net	2,316,988	2,192,262	2,142,074
Non Utility Plant
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	69,449	65,372	64,929
Work in progress	5,505	4,122	4,044
Less: Accumulated depreciation	18,548	17,598	17,284
Total property, plant, and equipment, net	$ 56,406	51,896	51,689
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	$ 16,000	500
Other non-current liabilities	$ 16,000	500
Current Headquarters Lease [Member]
Property, Plant and Equipment [Line Items]
Lease Expiration Date	Jan. 01, 2020
Gill Ranch [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Capital expenditures in accrued liabilities (2)	$ 300	$ 200	$ 100