Summary of Significant Accounting Policies, Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Accounting Policies [Abstract]
Regulated utility plants	$ 3,768,322	$ 3,759,541
Other	266,764	264,114
Property, plant and equipment	4,035,086	4,023,655
Accumulated depreciation	(1,252,575)	(1,230,783)
Net property, plant and equipment	$ 2,782,511	$ 2,792,872