Property, Plant and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	$ 3,261,886	$ 3,215,451	$ 3,247,177
Less: Accumulated depreciation	972,776	960,477	960,336
Total property, plant, and equipment, net	2,289,110	2,254,974	2,286,841
Capital expenditures in accrued liabilities	21,923	34,976	11,564
Other non-current liabilities	149,126	147,335	144,469
Utility Plant
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	3,003,962	2,975,217	2,867,271
Work in progress	177,133	159,924	76,631
Less: Accumulated depreciation	954,858	942,879	914,179
Total property, plant, and equipment, net	2,226,237	2,192,262	2,029,723
Non Utility Plant
Property, Plant and Equipment, Net, by Type [Abstract]
Property, plant and equipment, gross	76,436	75,639	299,324
Work in progress	4,355	4,671	3,951
Less: Accumulated depreciation	17,918	17,598	46,157
Total property, plant, and equipment, net	$ 62,873	62,712	$ 257,118
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	$ 4,100	500
Other non-current liabilities	$ 4,100	$ 500
Current Headquarters Lease [Member]
Property, Plant and Equipment [Line Items]
Lease Expiration Date	Jan. 01, 2020