Summary of Significant Accounting Policies, Property, Plant, and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Public Utility, Property, Plant, and Equipment [Line Items]
Total property, plant, and equipment	$ 4,185,832	$ 4,140,194
Accumulated depreciation	(1,332,760)	(1,311,273)
Net property, plant, and equipment	2,853,072	2,828,921
Unregulated Operation [Member]
Public Utility, Property, Plant, and Equipment [Line Items]
Other	268,607	268,254
Regulated Operation [Member]
Public Utility, Property, Plant, and Equipment [Line Items]
Regulated utility plants	$ 3,917,225	$ 3,871,940