Summary of Significant Accounting Policies, Property, Plant, and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Regulated utility plants	$ 4,601,293	$ 4,495,490
Other	15,515	13,470
Total property, plant, and equipment	4,616,808	4,508,960
Accumulated depreciation	(1,512,711)	(1,442,960)
Net property, plant, and equipment	$ 3,104,097	$ 3,066,000