Basis of Presentation and Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Summary Of Significant Accounting Policies [Line Items]
Property, plant and equipment (Note 4)	$ 3,747.7	$ 4,273.9
Deferred revenue	$ 14.2	12.2
Percentage of gross income from qualifying sources required to be subject to federal income tax, minimum	90.00%
Discount rate	15.00%
Granite Wash
Summary Of Significant Accounting Policies [Line Items]
Impaired assets to be disposed of	$ 8.5	13.2
Fair value disclosure	11.2
Impairment of intangible assets		20.0
Barnett
Summary Of Significant Accounting Policies [Line Items]
Impaired assets to be disposed of	354.4
Fair value disclosure	298.5
Impairment of intangible assets	238.9
Fayetteville
Summary Of Significant Accounting Policies [Line Items]
Impaired assets to be disposed of	61.9
Fair value disclosure	59.3
Impairment of intangible assets	70.9
Haynesville
Summary Of Significant Accounting Policies [Line Items]
Impaired assets to be disposed of	45.7
Fair value disclosure	3.8
Impairment of intangible assets	6.0
Watkins Glen
Summary Of Significant Accounting Policies [Line Items]
Impaired assets to be disposed of	31.2
Fair value disclosure	6.7
Natural Gas Liquids
Summary Of Significant Accounting Policies [Line Items]
Inventory	35.4	$ 37.5
Jackalope Gas Gathering Services, LLC
Summary Of Significant Accounting Policies [Line Items]
Equity method investment, other than temporary impairment	51.4
Equity method investment carrying value	202.4
Powder River Basin Industrial Complex, LLC
Summary Of Significant Accounting Policies [Line Items]
Equity method investment, other than temporary impairment	23.4
Equity method investment carrying value	$ 15.1
Minimum
Summary Of Significant Accounting Policies [Line Items]
Discount rate	10.00%
Maximum
Summary Of Significant Accounting Policies [Line Items]
Discount rate	16.00%