Summary of Significant Accounting Policies, Property, Plant, and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Public Utility, Property, Plant, and Equipment [Line Items]
Total property, plant, and equipment	$ 4,233,991	$ 4,140,194
Accumulated depreciation	(1,350,533)	(1,311,273)
Net property, plant, and equipment	2,883,458	2,828,921
Unregulated Operation [Member]
Public Utility, Property, Plant, and Equipment [Line Items]
Other	269,500	268,254
Regulated Operation [Member]
Public Utility, Property, Plant, and Equipment [Line Items]
Regulated utility plants	$ 3,964,491	$ 3,871,940