Significant Accounting Policies (Property Plant and Equipment) (Details Textuals) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Property, Plant and Equipment [Line Items]
Depreciation	$ 98.2	$ 77.8	$ 75.7
Interest Cost	$ 4.0	$ 0.9	$ 0.1
Transmission Assets [Member] | Useful Life Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	20 years
Transmission Assets [Member] | Useful Life Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	30 years
Gathering Assets [Member] | Useful Life Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	15 years
Gathering Assets [Member] | Useful Life Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	20 years
Gas Processing Plants [Member] | Useful Life Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	20 years
Other Property and Equipment [Member] | Useful Life Minimum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Other Property and Equipment [Member] | Useful Life Maximum [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	15 years