Guarantee of Securities of Subsidiaries (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Assets
Cash and cash equivalents	$ 53.9	$ 83.7	$ 53.9	$ 83.7
Receivables	102.6		102.6		106.2
Gas stored underground	10.0		10.0		3.6
Prepayments	16.0		16.0		11.4
Advances to affiliates	0		0		0
Other current assets	25.1		25.1		27.0
Total current assets	207.6		207.6		203.2
Investment in consolidated subsidiaries	0		0		0
Property, plant and equipment, gross	7,085.3		7,085.3		7,043.8
Less-accumulated depreciation and amortization	(888.4)		(888.4)		(785.8)
Property, plant and equipment, net	6,196.9		6,196.9		6,258.0
Other noncurrent assets	389.7		389.7		416.8
Advances to affiliates - noncurrent	0		0		0
Total other assets	389.7		389.7		416.8
Total Assets	6,794.2		6,794.2		6,878.0
Liabilities & Partners' Capital/Member's Equity
Payables	46.2		46.2		86.8
Advances from affiliates	0		0		0
Other current liabilities	144.3		144.3		127.0
Total current liabilities	190.5		190.5		213.8
Total long-term debt	3,122.3		3,122.3		3,252.3
Payable to affiliate	16.0		16.0		16.0
Other noncurrent liabilities	158.1		158.1		154.5
Total other liabilities and deferred credits	174.1		174.1		170.5
Total partners' capital/member's equity	3,307.3		3,307.3		3,241.4
Total Liabilities and Partners' Capital/Member's Equity	6,794.2		6,794.2		6,878.0
Operating revenues:
Gas transportation	244.3	227.9	534.8	499.8
Parking and lending	3.1	8.9	6.3	18.3
Gas storage	11.7	13.3	25.5	28.4
Other	2.9	6.6	6.4	10.7
Total operating revenues	262.0	256.7	573.0	557.2
Operating cost and expenses:
Fuel and gas transportation	28.2	24.2	52.0	54.1
Operation and maintenance	42.1	34.0	77.2	63.4
Administrative and general	33.7	29.9	70.8	65.6
Other operating costs and expenses	103.4	76.6	187.9	154.7
Total operating costs and expenses	207.4	164.7	387.9	337.8
Operating income	54.6	92.0	185.1	219.4
Other deductions (income):
Interest expense	37.6	35.7	75.9	70.9
Interest expense, affiliate, net	2.0	2.0	4.0	4.0
Loss on debt extinguishment	0	0	7.4	0
Interest income	(0.1)	(0.2)	(0.2)	(0.3)
Equity in earnings of subsidiaries	0	0	0	0
Miscellaneous other income	(0.1)	0	(0.4)	(0.1)
Total other deductions	39.4	37.5	86.7	74.5
Income before income taxes	15.2	54.5	98.4	144.9
Income taxes	0	0.1	0.2	0.2
Net Income	15.2	54.4	98.2	144.7
Statement of Cash Flows [Abstract]
Net Cash Provided by (Used In) Operating Activities			242.7	251.5
Investing Activities:
Capital expenditures			(73.7)	(130.5)
Proceeds from sale of operating assets			5.7	0
Advances to affiliates, net			0	(0.2)
Investment in (distributions from) consolidated subsidiary			0	0
Net cash used in investing activities			(68.0)	(130.7)
Financing Activities:
Proceeds from long-term debt, net of issuance costs			437.6	0
Repayment of borrowings from long-term debt			(135.0)	0
Payments on note payable			0	(0.3)
Payments associated with registration rights agreement			0	(10.7)
Payments of premiums on extinguishment of long-term debt			(11.8)	0
Proceeds from borrowings on revolving credit agreement			265.0	125.0
Repayment of borrowings on revolving credit agreement			(700.0)	0
Advances from affiliate			0	(0.1)
Proceeds from sale of common units			170.0	0
Capital contribution from general partner			3.6	0
Distributions paid			(205.2)	(196.8)
Net Cash (Used in) Provided by Financing Activities			(175.8)	(82.9)
(Decrease) increase in cash and cash equivalents			(1.1)	37.9
Cash and cash equivalents at beginning of period			55.0	45.8
Cash and Cash Equivalents at End of Period	53.9	83.7	53.9	83.7
Parent Guarantor [Member]
Assets
Cash and cash equivalents	0.3	0.2	0.3	0.2
Receivables	0		0		0
Gas stored underground	0		0		0
Prepayments	0		0		0
Advances to affiliates	0		0		0
Other current assets	0		0		0
Total current assets	0.3		0.3		0
Investment in consolidated subsidiaries	883.0		883.0		799.4
Property, plant and equipment, gross	0.6		0.6		0.6
Less-accumulated depreciation and amortization	(0.6)		(0.6)		(0.5)
Property, plant and equipment, net	0		0		0.1
Other noncurrent assets	0		0		0
Advances to affiliates - noncurrent	2,442.9		2,442.9		2,461.4
Total other assets	2,442.9		2,442.9		2,461.4
Total Assets	3,326.2		3,326.2		3,260.9
Liabilities & Partners' Capital/Member's Equity
Payables	2.7		2.7		3.5
Advances from affiliates	0		0		0
Other current liabilities	0.2		0.2		0
Total current liabilities	2.9		2.9		3.5
Total long-term debt	0		0		0
Payable to affiliate	16.0		16.0		16.0
Other noncurrent liabilities	0		0		0
Total other liabilities and deferred credits	16.0		16.0		16.0
Total partners' capital/member's equity	3,307.3		3,307.3		3,241.4
Total Liabilities and Partners' Capital/Member's Equity	3,326.2		3,326.2		3,260.9
Operating revenues:
Gas transportation	0	0	0	0
Parking and lending	0	0	0	0
Gas storage	0	0	0	0
Other	0	0	0	0
Total operating revenues	0	0	0	0
Operating cost and expenses:
Fuel and gas transportation	0	0	0	0
Operation and maintenance	0	0	0	0
Administrative and general	(0.1)	(0.1)	(0.2)	1.5
Other operating costs and expenses	0.1	0.1	0.2	0.2
Total operating costs and expenses	0	0	0	1.7
Operating income	0	0	0	(1.7)
Other deductions (income):
Interest expense	0	0.1	0	0
Interest expense, affiliate, net	(7.7)	(9.1)	(15.5)	(17.2)
Loss on debt extinguishment		0	0	0
Interest income	0	0	0	0
Equity in earnings of subsidiaries	(7.5)	(45.4)	(82.7)	(129.2)
Miscellaneous other income	0	0	0	0
Total other deductions	(15.2)	(54.4)	(98.2)	(146.4)
Income before income taxes	15.2	54.4	98.2	144.7
Income taxes	0	0	0	0
Net Income	15.2	54.4	98.2	144.7
Statement of Cash Flows [Abstract]
Net Cash Provided by (Used In) Operating Activities			14.8	145.2
Investing Activities:
Capital expenditures			0	0
Proceeds from sale of operating assets			0	0
Advances to affiliates, net			18.5	(4.7)
Investment in (distributions from) consolidated subsidiary			(1.4)	67.6
Net cash used in investing activities			17.1	62.9
Financing Activities:
Proceeds from long-term debt, net of issuance costs			0	0
Repayment of borrowings from long-term debt			0	0
Payments on note payable			0	(0.3)
Payments associated with registration rights agreement			0	(10.7)
Payments of premiums on extinguishment of long-term debt			0	0
Proceeds from borrowings on revolving credit agreement			0	0
Repayment of borrowings on revolving credit agreement			0	0
Contribution from parent			0	0
Advances from affiliate			0	(0.1)
Proceeds from sale of common units			170.0	0
Capital contribution from general partner			3.6	0
Distributions paid			(205.2)	(196.8)
Net Cash (Used in) Provided by Financing Activities			(31.6)	(207.9)
(Decrease) increase in cash and cash equivalents			0.3	0.2
Cash and cash equivalents at beginning of period			0	0
Cash and Cash Equivalents at End of Period	0.3	0.2	0.3	0.2
Subsidiary Issuer [Member]
Assets
Cash and cash equivalents	51.5	82.8	51.5	82.8
Receivables	0		0		0
Gas stored underground	0		0		0
Prepayments	0		0		0
Advances to affiliates	12.4		12.4		107.8
Other current assets	0		0		0
Total current assets	63.9		63.9		160.4
Investment in consolidated subsidiaries	5,006.0		5,006.0		4,940.9
Property, plant and equipment, gross	0		0		0
Less-accumulated depreciation and amortization	0		0		0
Property, plant and equipment, net	0		0		0
Other noncurrent assets	1.6		1.6		1.8
Advances to affiliates - noncurrent	0		0		0
Total other assets	1.6		1.6		1.8
Total Assets	5,071.5		5,071.5		5,103.1
Liabilities & Partners' Capital/Member's Equity
Payables	0		0		0.3
Advances from affiliates	1.5		1.5		0
Other current liabilities	15.5		15.5		15.5
Total current liabilities	17.0		17.0		15.8
Total long-term debt	1,219.7		1,219.7		1,464.3
Payable to affiliate	2,951.4		2,951.4		2,823.6
Other noncurrent liabilities	0		0		0
Total other liabilities and deferred credits	2,951.4		2,951.4		2,823.6
Total partners' capital/member's equity	883.4		883.4		799.4
Total Liabilities and Partners' Capital/Member's Equity	5,071.5		5,071.5		5,103.1
Operating revenues:
Gas transportation	0	0	0	0
Parking and lending	0	0	0	0
Gas storage	0	0	0	0
Other	0	0	0	0
Total operating revenues	0	0	0	0
Operating cost and expenses:
Fuel and gas transportation	0	0	0	0
Operation and maintenance	0	0	0	0
Administrative and general	0	0	0	0
Other operating costs and expenses	0	0	0	0
Total operating costs and expenses	0	0	0	0
Operating income	0	0	0	0
Other deductions (income):
Interest expense	16.2	16.3	32.4	32.2
Interest expense, affiliate, net	11.2	11.2	22.2	21.0
Loss on debt extinguishment		0	0	0
Interest income	0	0	0	0
Equity in earnings of subsidiaries	(36.0)	(72.9)	(139.3)	(182.4)
Miscellaneous other income	0	0	0	0
Total other deductions	(8.6)	(45.4)	(84.7)	(129.2)
Income before income taxes	8.6	45.4	84.7	129.2
Income taxes	0	0	0	0
Net Income	8.6	45.4	84.7	129.2
Statement of Cash Flows [Abstract]
Net Cash Provided by (Used In) Operating Activities			19.1	(49.4)
Investing Activities:
Capital expenditures			0	0
Proceeds from sale of operating assets			0	0
Advances to affiliates, net			95.4	72.9
Investment in (distributions from) consolidated subsidiary			0	0
Net cash used in investing activities			95.4	72.9
Financing Activities:
Proceeds from long-term debt, net of issuance costs			0	0
Repayment of borrowings from long-term debt			0	0
Payments on note payable			0	0
Payments associated with registration rights agreement			0	0
Payments of premiums on extinguishment of long-term debt			0	0
Proceeds from borrowings on revolving credit agreement			115.0	125.0
Repayment of borrowings on revolving credit agreement			(360.0)	0
Contribution from parent			0	0
Advances from affiliate			129.4	85.5
Proceeds from sale of common units			0	0
Capital contribution from general partner			0	0
Distributions paid			0	(196.8)
Net Cash (Used in) Provided by Financing Activities			(115.6)	13.7
(Decrease) increase in cash and cash equivalents			(1.1)	37.2
Cash and cash equivalents at beginning of period			52.6	45.6
Cash and Cash Equivalents at End of Period	51.5	82.8	51.5	82.8
Non-Guarantor Subsidiaries [Member]
Assets
Cash and cash equivalents	2.1	0.7	2.1	0.7
Receivables	109.9		109.9		115.2
Gas stored underground	10.0		10.0		3.6
Prepayments	16.0		16.0		11.4
Advances to affiliates	1.5		1.5		0
Other current assets	34.1		34.1		34.9
Total current assets	173.6		173.6		167.5
Investment in consolidated subsidiaries	0		0		0
Property, plant and equipment, gross	7,084.7		7,084.7		7,043.2
Less-accumulated depreciation and amortization	(887.8)		(887.8)		(785.3)
Property, plant and equipment, net	6,196.9		6,196.9		6,257.9
Other noncurrent assets	388.4		388.4		417.0
Advances to affiliates - noncurrent	508.5		508.5		362.2
Total other assets	896.9		896.9		779.2
Total Assets	7,267.4		7,267.4		7,204.6
Liabilities & Partners' Capital/Member's Equity
Payables	50.8		50.8		101.3
Advances from affiliates	12.4		12.4		107.8
Other current liabilities	137.6		137.6		112.2
Total current liabilities	200.8		200.8		321.3
Total long-term debt	1,902.6		1,902.6		1,788.0
Payable to affiliate	0		0		0
Other noncurrent liabilities	158.4		158.4		154.4
Total other liabilities and deferred credits	158.4		158.4		154.4
Total partners' capital/member's equity	5,005.6		5,005.6		4,940.9
Total Liabilities and Partners' Capital/Member's Equity	7,267.4		7,267.4		7,204.6
Operating revenues:
Gas transportation	267.7	253.7	583.0	554.7
Parking and lending	3.1	9.6	7.1	20.0
Gas storage	11.7	13.3	25.5	28.4
Other	2.9	6.6	6.4	10.7
Total operating revenues	285.4	283.2	622.0	613.8
Operating cost and expenses:
Fuel and gas transportation	51.6	50.7	101.0	110.7
Operation and maintenance	42.1	34.0	77.2	63.4
Administrative and general	33.8	30.0	71.0	64.1
Other operating costs and expenses	103.3	76.5	187.7	154.5
Total operating costs and expenses	230.8	191.2	436.9	392.7
Operating income	54.6	92.0	185.1	221.1
Other deductions (income):
Interest expense	21.4	19.3	43.5	38.7
Interest expense, affiliate, net	(1.5)	(0.1)	(2.7)	0.2
Loss on debt extinguishment		0	7.4	0
Interest income	(0.1)	(0.2)	(0.2)	(0.3)
Equity in earnings of subsidiaries	0	0	0	0
Miscellaneous other income	(0.1)	0	(0.4)	(0.1)
Total other deductions	19.7	19.0	47.6	38.5
Income before income taxes	34.9	73.0	137.5	182.6
Income taxes	0	0.1	0.2	0.2
Net Income	34.9	72.9	137.3	182.4
Statement of Cash Flows [Abstract]
Net Cash Provided by (Used In) Operating Activities			281.2	284.9
Investing Activities:
Capital expenditures			(73.7)	(130.5)
Proceeds from sale of operating assets			5.7	0
Advances to affiliates, net			(147.9)	(81.0)
Investment in (distributions from) consolidated subsidiary			0	0
Net cash used in investing activities			(215.9)	(211.5)
Financing Activities:
Proceeds from long-term debt, net of issuance costs			437.6	0
Repayment of borrowings from long-term debt			(135.0)	0
Payments on note payable			0	0
Payments associated with registration rights agreement			0	0
Payments of premiums on extinguishment of long-term debt			(11.8)	0
Proceeds from borrowings on revolving credit agreement			150.0	0
Repayment of borrowings on revolving credit agreement			(340.0)	0
Contribution from parent			1.4	0
Advances from affiliate			(95.4)	(72.9)
Proceeds from sale of common units			0	0
Capital contribution from general partner			0	0
Distributions paid			(72.4)	0
Net Cash (Used in) Provided by Financing Activities			(65.6)	(72.9)
(Decrease) increase in cash and cash equivalents			(0.3)	0.5
Cash and cash equivalents at beginning of period			2.4	0.2
Cash and Cash Equivalents at End of Period	2.1	0.7	2.1	0.7
Eliminations [Member]
Assets
Cash and cash equivalents	0	0	0	0
Receivables	(7.3)		(7.3)		(9.0)
Gas stored underground	0		0		0
Prepayments	0		0		0
Advances to affiliates	(13.9)		(13.9)		(107.8)
Other current assets	(9.0)		(9.0)		(7.9)
Total current assets	(30.2)		(30.2)		(124.7)
Investment in consolidated subsidiaries	(5,889.0)		(5,889.0)		(5,740.3)
Property, plant and equipment, gross	0		0		0
Less-accumulated depreciation and amortization	0		0		0
Property, plant and equipment, net	0		0		0
Other noncurrent assets	(0.3)		(0.3)		(2.0)
Advances to affiliates - noncurrent	(2,951.4)		(2,951.4)		(2,823.6)
Total other assets	(2,951.7)		(2,951.7)		(2,825.6)
Total Assets	(8,870.9)		(8,870.9)		(8,690.6)
Liabilities & Partners' Capital/Member's Equity
Payables	(7.3)		(7.3)		(18.3)
Advances from affiliates	(13.9)		(13.9)		(107.8)
Other current liabilities	(9.0)		(9.0)		(0.7)
Total current liabilities	(30.2)		(30.2)		(126.8)
Total long-term debt	0		0		0
Payable to affiliate	(2,951.4)		(2,951.4)		(2,823.6)
Other noncurrent liabilities	(0.3)		(0.3)		0.1
Total other liabilities and deferred credits	(2,951.7)		(2,951.7)		(2,823.5)
Total partners' capital/member's equity	(5,889.0)		(5,889.0)		(5,740.3)
Total Liabilities and Partners' Capital/Member's Equity	(8,870.9)		(8,870.9)		(8,690.6)
Operating revenues:
Gas transportation	(23.4)	(25.8)	(48.2)	(54.9)
Parking and lending	0	(0.7)	(0.8)	(1.7)
Gas storage	0	0	0	0
Other	0	0	0	0
Total operating revenues	(23.4)	(26.5)	(49.0)	(56.6)
Operating cost and expenses:
Fuel and gas transportation	(23.4)	(26.5)	(49.0)	(56.6)
Operation and maintenance	0	0	0	0
Administrative and general	0	0	0	0
Other operating costs and expenses	0	0	0	0
Total operating costs and expenses	(23.4)	(26.5)	(49.0)	(56.6)
Operating income	0	0	0	0
Other deductions (income):
Interest expense	0	0	0	0
Interest expense, affiliate, net	0	0	0	0
Loss on debt extinguishment		0	0	0
Interest income	0	0	0	0
Equity in earnings of subsidiaries	43.5	118.3	222.0	311.6
Miscellaneous other income	0	0	0	0
Total other deductions	43.5	118.3	222.0	311.6
Income before income taxes	(43.5)	(118.3)	(222.0)	(311.6)
Income taxes	0	0	0	0
Net Income	(43.5)	(118.3)	(222.0)	(311.6)
Statement of Cash Flows [Abstract]
Net Cash Provided by (Used In) Operating Activities			(72.4)	(129.2)
Investing Activities:
Capital expenditures			0	0
Proceeds from sale of operating assets			0	0
Advances to affiliates, net			34.0	12.6
Investment in (distributions from) consolidated subsidiary			1.4	(67.6)
Net cash used in investing activities			35.4	(55.0)
Financing Activities:
Proceeds from long-term debt, net of issuance costs			0	0
Repayment of borrowings from long-term debt			0	0
Payments on note payable			0	0
Payments associated with registration rights agreement			0	0
Payments of premiums on extinguishment of long-term debt			0	0
Proceeds from borrowings on revolving credit agreement			0	0
Repayment of borrowings on revolving credit agreement			0	0
Contribution from parent			(1.4)	0
Advances from affiliate			(34.0)	(12.6)
Proceeds from sale of common units			0	0
Capital contribution from general partner			0	0
Distributions paid			72.4	196.8
Net Cash (Used in) Provided by Financing Activities			37.0	184.2
(Decrease) increase in cash and cash equivalents			0	0
Cash and cash equivalents at beginning of period			0	0
Cash and Cash Equivalents at End of Period	$ 0	$ 0	$ 0	$ 0