Summary of Significant Accounting Policies (Utility Plant) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Public Utility, Property, Plant and Equipment [Line Items]
Capitalized interest costs	$ 2,807	$ 2,188	$ 1,188
Construction in progress	45,851	70,453
Major components of depreciable plant and equipment [Abstract]
Equipment	307,938	269,734
Transmission and distribution	1,295,690	1,204,520
Office buildings and other structures	110,600	79,762
Total depreciable plant and equipment	$ 1,714,228	$ 1,554,016
Estimated Useful Lives of Plant and Equipment [Abstract]
Depreciation expense as percentage of beginning of year balance of depreciable plant	3.60%	3.50%	3.40%
Minimum [Member]
Estimated Useful Lives of Plant and Equipment [Abstract]
Estimated useful life of equipment	5 years
Estimated useful life of transmission and distribution plant	35 years
Estimated useful lives of office buildings and other structures	7 years
Estimated service lives of assets	5 years
Useful life of intangible assets	5 years
Maximum [Member]
Estimated Useful Lives of Plant and Equipment [Abstract]
Estimated useful life of equipment	35 years
Estimated useful life of transmission and distribution plant	75 years
Estimated useful lives of office buildings and other structures	50 years
Estimated service lives of assets	75 years
Useful life of intangible assets	70 years
General and Administrative Expense [Member]
Estimated Useful Lives of Plant and Equipment [Abstract]
Depreciation	$ 2,209	$ 1,670	$ 1,590
Water Plant [Member]
Estimated Useful Lives of Plant and Equipment [Abstract]
Depreciation	$ 46,456	$ 42,659	$ 38,722