Guarantee of Securities of Subsidiaries Balance Sheets (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Feb. 01, 2012	Dec. 31, 2011
Assets
Cash and cash equivalents	$ 4.3	$ 3.9	$ 3.9	$ 3.9	$ 21.9
Receivables	109.3	112.2
Gas stored underground	0.9	7.0
Prepayments	11.7	15.2
Advances to affiliates		0
Other current assets	14.0	14.9
Total current assets	140.2	153.2
Investment in consolidated subsidiaries	0	0
Property, plant and equipment, gross	8,470.5	8,423.3
Less-accumulated depreciation and amortization	1,283.4	1,234.1
Property, plant and equipment, net	7,187.1	7,189.2
Other noncurrent assets	516.6	520.1
Advances to affiliates, noncurrent	0	0
Total other assets	516.6	520.1
Total Assets	7,843.9	7,862.5
Liabilities and Equity [Abstract]
Payables	60.6	89.0
Payable to affiliates	3.0	2.7
Other current liabilities	129.8	164.1
Total current liabilities	193.4	255.8
Total long-term debt	3,607.7	3,539.2
Payable to affiliate	16.0	16.0
Other noncurrent liabilities	182.7	174.4
Total other liabilities and deferred credits	198.7	190.4
Total partners' capital/member's equity	3,844.1	3,877.1
Total Liabilities and Partners' Capital	7,843.9	7,862.5
Parent Guarantor [Member]
Assets
Cash and cash equivalents	0	0.1	0		0.5
Receivables	0	0
Gas stored underground	0	0
Prepayments	0.4	0
Advances to affiliates		0
Other current assets	0.2	0.4
Total current assets	0.6	0.5
Investment in consolidated subsidiaries	1,343.9	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	0.1	0.1
Advances to affiliates, noncurrent	2,519.4	2,638.5
Total other assets	2,519.5	2,638.6
Total Assets	3,864.0	3,896.1
Liabilities and Equity [Abstract]
Payables	0.5	0.1
Payable to affiliates	3.0	2.7
Other current liabilities	0.4	0.2
Total current liabilities	3.9	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	3,844.1	3,877.1
Total Liabilities and Partners' Capital	3,864.0	3,896.1
Subsidiary Issuer [Member]
Assets
Cash and cash equivalents	2.0	1.0	1.9		10.7
Receivables	0	0
Gas stored underground	0	0
Prepayments	0	0
Advances to affiliates		0
Other current assets	0	0
Total current assets	2.0	1.0
Investment in consolidated subsidiaries	5,911.7	5,785.7
Property, plant and equipment, gross	0.3	0
Less-accumulated depreciation and amortization	0	0
Property, plant and equipment, net	0.3	0
Other noncurrent assets	4.5	4.8
Advances to affiliates, noncurrent	115.7	84.4
Total other assets	120.2	89.2
Total Assets	6,034.2	5,875.9
Liabilities and Equity [Abstract]
Payables	0.2	0
Payable to affiliates	0	2.0
Other current liabilities	16.3	16.9
Total current liabilities	16.5	18.9
Total long-term debt	1,379.1	1,378.9
Payable to affiliate	3,294.7	3,221.1
Other noncurrent liabilities	0	0
Total other liabilities and deferred credits	3,294.7	3,221.1
Total partners' capital/member's equity	1,343.9	1,257.0
Total Liabilities and Partners' Capital	6,034.2	5,875.9
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	2.3	2.8	2.0		10.7
Receivables	116.3	119.5
Gas stored underground	0.9	7.0
Prepayments	11.3	15.2
Advances to affiliates		2.0
Other current assets	20.6	18.1
Total current assets	151.4	164.6
Investment in consolidated subsidiaries	0	0
Property, plant and equipment, gross	8,469.6	8,422.7
Less-accumulated depreciation and amortization	1,282.8	1,233.5
Property, plant and equipment, net	7,186.8	7,189.2
Other noncurrent assets	512.0	515.2
Advances to affiliates, noncurrent	775.3	582.6
Total other assets	1,287.3	1,097.8
Total Assets	8,625.5	8,451.6
Liabilities and Equity [Abstract]
Payables	66.9	96.2
Payable to affiliates	0	0
Other current liabilities	119.9	150.4
Total current liabilities	186.8	246.6
Total long-term debt	2,228.6	2,160.3
Payable to affiliate	115.7	84.4
Other noncurrent liabilities	182.7	174.6
Total other liabilities and deferred credits	298.4	259.0
Total partners' capital/member's equity	5,911.7	5,785.7
Total Liabilities and Partners' Capital	8,625.5	8,451.6
Consolidation, Eliminations [Member]
Assets
Cash and cash equivalents	0	0	0		0
Receivables	(7.0)	(7.3)
Gas stored underground	0	0
Prepayments	0	0
Advances to affiliates		(2.0)
Other current assets	(6.8)	(3.6)
Total current assets	(13.8)	(12.9)
Investment in consolidated subsidiaries	(7,255.6)	(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	0	0
Advances to affiliates, noncurrent	(3,410.4)	(3,305.5)
Total other assets	(3,410.4)	(3,305.5)
Total Assets	(10,679.8)	(10,361.1)
Liabilities and Equity [Abstract]
Payables	(7.0)	(7.3)
Payable to affiliates	0	(2.0)
Other current liabilities	(6.8)	(3.4)
Total current liabilities	(13.8)	(12.7)
Total long-term debt	0	0
Payable to affiliate	(3,410.4)	(3,305.5)
Other noncurrent liabilities	0	(0.2)
Total other liabilities and deferred credits	(3,410.4)	(3,305.7)
Total partners' capital/member's equity	(7,255.6)	(7,042.7)
Total Liabilities and Partners' Capital	$ (10,679.8)	$ (10,361.1)