Guarantee of Securities of Subsidiaries Balance Sheets (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Assets
Cash and cash equivalents	$ 9.9	$ 3.9	$ 11.4	$ 21.9
Receivables	94.7	112.2
Gas and liquids stored underground	1.6	10.8
Prepayments	18.3	15.2
Advances to affiliates		0
Other current assets	14.7	14.9
Total current assets	139.2	157.0
Investment in consolidated subsidiaries	0	0
Property, plant and equipment, gross	8,522.6	8,423.3
Less-accumulated depreciation and amortization	1,345.9	1,234.1
Property, plant and equipment, net	7,176.7	7,189.2
Other noncurrent assets	506.0	516.3
Advances to affiliates - noncurrent	0	0
Investment in unconsolidated affiliates	25.0	0
Total other assets	531.0	516.3
Total Assets	7,846.9	7,862.5
Liabilities and Equity [Abstract]
Payables	48.6	89.0
Payable to affiliates	1.0	2.7
Other current liabilities	149.3	164.1
Total current liabilities	198.9	255.8
Total long-term debt	3,258.3	3,539.2
Payable to affiliate	16.0	16.0
Other noncurrent liabilities	184.3	174.4
Total other liabilities and deferred credits	200.3	190.4
Total partners' capital/member's equity	4,169.1	3,877.1
Noncontrolling interest	20.3	0
Total Equity	4,189.4	3,877.1	3,371.6	3,487.0
Total Liabilities and Equity	7,846.9	7,862.5
Parent Guarantor [Member]
Assets
Cash and cash equivalents	0	0.1	0	0.5
Receivables	0	0
Gas and liquids stored underground	0	0
Prepayments	0.2	0
Advances to affiliates		0
Other current assets	0.2	0.4
Total current assets	0.4	0.5
Investment in consolidated subsidiaries	1,412.4	1,257.0
Property, plant and equipment, gross	0.6	0.6
Less-accumulated depreciation and amortization	0.6	0.6
Property, plant and equipment, net	0	0
Other noncurrent assets	0	0.1
Advances to affiliates - noncurrent	2,773.9	2,638.5
Investment in unconsolidated affiliates	0
Total other assets	2,773.9	2,638.6
Total Assets	4,186.7	3,896.1
Liabilities and Equity [Abstract]
Payables	0.3	0.1
Payable to affiliates	1.0	2.7
Other current liabilities	0.3	0.2
Total current liabilities	1.6	3.0
Total long-term debt	0	0
Payable to affiliate	16.0	16.0
Other noncurrent liabilities	0	0
Total other liabilities and deferred credits	16.0	16.0
Total partners' capital/member's equity	4,169.1	3,877.1
Noncontrolling interest	0	0
Total Equity	4,169.1	3,877.1
Total Liabilities and Equity	4,186.7	3,896.1
Subsidiary Issuer [Member]
Assets
Cash and cash equivalents	1.2	1.0	3.1	10.7
Receivables	2.4	0
Gas and liquids stored underground	0	0
Prepayments	0.1	0
Advances to affiliates		0
Other current assets	0	0
Total current assets	3.7	1.0
Investment in consolidated subsidiaries	6,012.3	5,785.7
Property, plant and equipment, gross	0	0
Less-accumulated depreciation and amortization	0	0
Property, plant and equipment, net	0	0
Other noncurrent assets	4.3	4.8
Advances to affiliates - noncurrent	140.9	84.4
Investment in unconsolidated affiliates	0
Total other assets	145.2	89.2
Total Assets	6,161.2	5,875.9
Liabilities and Equity [Abstract]
Payables	0	0
Payable to affiliates	0	2.0
Other current liabilities	22.0	16.9
Total current liabilities	22.0	18.9
Total long-term debt	1,379.4	1,378.9
Payable to affiliate	3,347.4	3,221.1
Other noncurrent liabilities	0	0
Total other liabilities and deferred credits	3,347.4	3,221.1
Total partners' capital/member's equity	1,412.4	1,257.0
Noncontrolling interest	0	0
Total Equity	1,412.4	1,257.0
Total Liabilities and Equity	6,161.2	5,875.9
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	8.7	2.8	8.3	10.7
Receivables	99.3	119.5
Gas and liquids stored underground	1.6	10.8
Prepayments	18.0	15.2
Advances to affiliates		2.0
Other current assets	24.2	18.1
Total current assets	151.8	168.4
Investment in consolidated subsidiaries	0	0
Property, plant and equipment, gross	8,522.0	8,422.7
Less-accumulated depreciation and amortization	1,345.3	1,233.5
Property, plant and equipment, net	7,176.7	7,189.2
Other noncurrent assets	500.3	511.4
Advances to affiliates - noncurrent	573.5	582.6
Investment in unconsolidated affiliates	25.0
Total other assets	1,098.8	1,094.0
Total Assets	8,427.3	8,451.6
Liabilities and Equity [Abstract]
Payables	48.3	96.2
Payable to affiliates	7.0	0
Other current liabilities	135.0	150.4
Total current liabilities	190.3	246.6
Total long-term debt	1,878.9	2,160.3
Payable to affiliate	140.9	84.4
Other noncurrent liabilities	184.6	174.6
Total other liabilities and deferred credits	325.5	259.0
Total partners' capital/member's equity	6,012.3	5,785.7
Noncontrolling interest	20.3	0
Total Equity	6,032.6	5,785.7
Total Liabilities and Equity	8,427.3	8,451.6
Consolidation, Eliminations [Member]
Assets
Cash and cash equivalents	0	0	0	0
Receivables	(7.0)	(7.3)
Gas and liquids stored underground	0	0
Prepayments	0	0
Advances to affiliates		(2.0)
Other current assets	(9.7)	(3.6)
Total current assets	(16.7)	(12.9)
Investment in consolidated subsidiaries	(7,424.7)	(7,042.7)
Property, plant and equipment, gross	0	0
Less-accumulated depreciation and amortization	0	0
Property, plant and equipment, net	0	0
Other noncurrent assets	1.4	0
Advances to affiliates - noncurrent	(3,488.3)	(3,305.5)
Investment in unconsolidated affiliates	0
Total other assets	(3,486.9)	(3,305.5)
Total Assets	(10,928.3)	(10,361.1)
Liabilities and Equity [Abstract]
Payables	0	(7.3)
Payable to affiliates	(7.0)	(2.0)
Other current liabilities	(8.0)	(3.4)
Total current liabilities	(15.0)	(12.7)
Total long-term debt	0	0
Payable to affiliate	(3,488.3)	(3,305.5)
Other noncurrent liabilities	(0.3)	(0.2)
Total other liabilities and deferred credits	(3,488.6)	(3,305.7)
Total partners' capital/member's equity	(7,424.7)	(7,042.7)
Noncontrolling interest	0	0
Total Equity	(7,424.7)	(7,042.7)
Total Liabilities and Equity	$ (10,928.3)	$ (10,361.1)