Summary of Significant Accounting Policies, Property, Plant, and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Regulated utility plants	$ 4,675,788	$ 4,645,698
Other	15,657	15,514
Total property, plant, and equipment	4,691,445	4,661,212
Accumulated depreciation	(1,562,348)	(1,536,158)
Net property, plant, and equipment	$ 3,129,097	$ 3,125,054