Financing (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended			6 Months Ended			6 Months Ended		9 Months Ended		6 Months Ended					6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Mar. 31, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jun. 30, 2011 Senior notes due 2021 [Member]	Jun. 30, 2011 Texas Gas 2013 Notes[Member]	Sep. 30, 2011 Texas Gas 2013 Notes[Member]	Jun. 30, 2011 Subordinated Debt [Member]	Jun. 30, 2011 Line of Credit [Member]	Jul. 31, 2011 Line of Credit [Member]	Dec. 31, 2010 Line of Credit [Member]	Jun. 30, 2011 Notes Payable, Other Payables [Member]	Dec. 31, 2010 Notes Payable, Other Payables [Member]	Jun. 30, 2011 Common Units [Member]
Debt Instrument [Line Items]
Net proceeds on issuance of debt							$ 437.6
Initial purchaser discounts and offering expenses on issuance of debt							4.0
Face amount of debt							325.0			200.0
Interest rate on debt (in hundredths)							4.50%	5.50%		8.00%
Maturity date of debt							Feb. 01, 2021	Apr. 01, 2013			Jun. 29, 2012
Amount of debt redeemed								135.0	115.0
Early redemption premium paid								11.8	9.2
Unamortized discounts and deferred offering costs related to redeemed notes								0.6
Amount of premium and unamortized discounts deferred due to application of regulatory accounting								5.0
Loss on the early extinguishment of debt recognized in the period	0	0		7.4	0			7.4	5.8
Outstanding balance										100.0				2.8	2.5
Weighted-average interest rate (in hundredths)											0.43%		0.53%	5.72%	5.89%
Aggregate lending commitments											950.0		950.0
Outstanding under credit facility											268.5	358.5	703.5
Available borrowing capacity											681.5	591.5
Additional borrowing							115.0
Financial covenants on credit facility											The financial covenants under the credit facility require the Partnership and its subsidiaries to maintain, among other things, a ratio of total consolidated debt to consolidated EBITDA (as defined in the credit agreement) measured for the previous twelve months of not more than 5.0 to 1.0.
Number of common units registered for resale under Amended and Restated Registration Rights Agreement (in number of shares)	27.9			27.9
Number of common units included in offering (in number of units)																6.0
Price per unit of public offering																$ 29.33
Proceeds from public offering																173.6
Amount of underwriting discounts and commissions																6.0
Contribution from general partner to maintain ownership percentage																3.6
Maximum amount Partnership must pay for reimbursement of underwriting discounts and commissions (in dollars per share)	$ 0.914			$ 0.914
Number of units sold in secondary offering (in shares)			11.5
Amount of underwriting discounts and commissions paid by Partnership			10.5
Other offering costs incurred by Partnership			0.2
Accrued liability for future underwriting discounts and commissions	$ 16.0			$ 16.0		$ 16.0