Summary of Significant Accounting Policies (Utility Plant) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Public Utility, Property, Plant and Equipment [Line Items]
Capitalized interest costs	$ 2,856	$ 2,807	$ 2,188
Construction in progress	68,765	45,851
Major components of depreciable plant and equipment [Abstract]
Equipment	335,358	307,938
Transmission and distribution	1,375,821	1,295,690
Office buildings and other structures	121,872	110,600
Total depreciable plant and equipment	$ 1,833,051	$ 1,714,228
Estimated Useful Lives of Plant and Equipment [Abstract]
Depreciation expense as percentage of beginning of year balance of depreciable plant	3.60%	3.60%	3.50%
Minimum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful life	7 years
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]
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful life	39 years
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
Administrative and general expense
Estimated Useful Lives of Plant and Equipment [Abstract]
Depreciation	$ 2,306	$ 2,209	$ 1,670
Water Plant [Member]
Estimated Useful Lives of Plant and Equipment [Abstract]
Depreciation	$ 53,031	$ 46,456	$ 42,659
Building and Building Improvements [Member] | Minimum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful life	7 years
Building and Building Improvements [Member] | Maximum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful life	39 years
Finite-Lived Intangible Assets [Member] | Minimum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful life	7 years
Finite-Lived Intangible Assets [Member] | Maximum [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Estimated useful life	12 years