Business Segments (Business Segments) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011		Dec. 31, 2011
Segment Reporting Information [Line Items]
Total operating revenue	$ 330.9		$ 593.6	[1]	$ 1,270.2	[2]	$ 1,803.1	[3]
Total purchases	(268.0)		(449.0)	[1]	(973.4)	[2]	(1,464.3)	[3]
Gross margin	62.9	[4]	144.6	[4]	296.8	[4]	338.8	[4]
Operating and maintenance expense	(35.7)		(36.7)	[1]	(91.7)	[2]	(91.3)	[3]
Depreciation and amortization expense	(14.8)		(25.9)	[1]	(49.6)	[2]	(74.9)	[3]
General and administrative expense	(11.1)		(12.0)	[1]	(34.0)	[2]	(35.2)	[3]
Other income	0.1		0.2	[1]	0.4	[2]	0.4	[3]
Earnings from unconsolidated affiliates	8.9		6.9	[1]	16.6	[2]	17.1	[3]
Interest expense, net	(8.1)		(8.6)	[1]	(31.8)	[2]	(25.0)	[3]
Income tax expense	(0.3)	[5]	(0.4)	[1],[5]	(1.0)	[2],[5]	(0.9)	[3],[5]
Net income	1.9		68.1	[1]	105.7	[2]	129.0	[3]
Net income attributable to noncontrolling interests	(0.6)		0.4	[1]	(2.0)	[2]	(12.8)	[3]
Net income (loss) attributable to partners	1.3		68.5	[1]	103.7	[2]	116.2	[3]
Non-cash derivative mark-to-market	(22.5)	[6]	60.4	[6]	18.9	[6]	47.3	[6]
Capital expenditures					152.5	[2]	98.5	[7]
Acquisition expenditures					405.2		174.9
Investments in unconsolidated affiliates					86.3	[2]	6.8	[7]
Total long-term assets	2,246.8				2,246.8				1,923.7
Current assets	270.6				270.6				353.7	[8]
Total assets	2,517.4				2,517.4				2,277.4	[8]
Natural Gas Services [Member]
Segment Reporting Information [Line Items]
Total operating revenue	278.5	[9]	478.8	[9]	910.4	[9]	1,310.9	[9]
Total purchases	(232.9)	[9]	(354.9)	[9]	(683.6)	[9]	(1,043.7)	[9]
Gross margin	45.6	[4],[9]	123.9	[4],[9]	226.8	[4],[9]	267.2	[4],[9]
Operating and maintenance expense	(26.9)	[9]	(28.0)	[9]	(67.8)	[9]	(69.0)	[9]
Depreciation and amortization expense	(12.5)	[9]	(22.8)	[9]	(43.0)	[9]	(66.7)	[9]
Earnings from unconsolidated affiliates	3.9	[9]	6.9	[9]	11.6	[9]	17.1	[9]
Net income	10.1	[9]	80.0	[9]	127.6	[9]	148.6	[9]
Net income attributable to noncontrolling interests	(0.6)	[9]	0.4	[9]	(2.0)	[9]	(12.8)	[9]
Net income (loss) attributable to partners	9.5	[9]	80.4	[9]	125.6	[9]	135.8	[9]
Non-cash derivative mark-to-market	(20.8)	[6],[9]	61.0	[6],[9]	5.4	[6],[9]	49.7	[6],[9]
Capital expenditures					141.5	[9]	88.8	[9]
Acquisition expenditures					375.4	[9]	145.3	[9]
Investments in unconsolidated affiliates					52.7	[9]	6.8	[9]
Total long-term assets	1,768.1	[9]			1,768.1	[9]			1,555.4	[9]
NGL Logistics [Member]
Segment Reporting Information [Line Items]
Total operating revenue	15.8		14.7		46.2		42.3
Total purchases							(4.7)
Gross margin	15.8	[4]	14.7	[4]	46.2	[4]	37.6	[4]
Operating and maintenance expense	(5.1)		(5.5)		(12.8)		(11.3)
Depreciation and amortization expense	(1.6)		(2.4)		(4.6)		(6.1)
Other income	0.1		0.2		0.4		0.4
Earnings from unconsolidated affiliates	5.0				5.0
Net income	14.2		7.0		34.2		20.6
Net income (loss) attributable to partners	14.2		7.0		34.2		20.6
Non-cash derivative mark-to-market				[6]
Capital expenditures					8.5		6.9
Acquisition expenditures					29.8		29.6
Investments in unconsolidated affiliates					33.6
Total long-term assets	331.0				331.0				250.1
Wholesale Propane Logistics [Member]
Segment Reporting Information [Line Items]
Total operating revenue	36.7		100.1		313.8		452.1
Total purchases	(35.2)		(94.1)		(290.0)		(418.1)
Gross margin	1.5	[4]	6.0	[4]	23.8	[4]	34.0	[4]
Operating and maintenance expense	(3.7)		(3.2)		(11.1)		(11.0)
Depreciation and amortization expense	(0.6)		(0.7)		(1.9)		(2.1)
Net income	(2.8)		2.1		10.8		20.9
Net income (loss) attributable to partners	(2.8)		2.1		10.8		20.9
Non-cash derivative mark-to-market	(2.1)	[6]	0.1	[6]	13.9	[6]	(0.7)	[6]
Capital expenditures					2.5		2.8
Total long-term assets	104.5				104.5				104.2
Other [Member]
Segment Reporting Information [Line Items]
Depreciation and amortization expense	(0.1)				(0.1)
General and administrative expense	(11.1)		(12.0)		(34.0)		(35.2)
Interest expense, net	(8.1)		(8.6)		(31.8)		(25.0)
Income tax expense	(0.3)	[5]	(0.4)	[5]	(1.0)	[5]	(0.9)	[5]
Net income	(19.6)		(21.0)		(66.9)		(61.1)
Net income (loss) attributable to partners	(19.6)		(21.0)		(66.9)		(61.1)
Non-cash derivative mark-to-market	0.4	[6]	(0.7)	[6]	(0.4)	[6]	(1.7)	[6]
Total long-term assets	43.2	[10]			43.2	[10]			14.0	[10]
Eliminations [Member]
Segment Reporting Information [Line Items]
Total operating revenue	(0.1)	[11]			(0.2)	[11]	(2.2)	[11]
Total purchases	$ 0.1	[11]			$ 0.2	[11]	$ 2.2	[11]

[1] The financial information for the three months ended September 30, 2011 includes the results of Southeast Texas, a transfer of net assets between entities under common control that was accounted for as if the transfer occurred at the beginning of the period, and prior years are retrospectively adjusted to furnish comparative information similar to the pooling method.

[2] The financial information for the nine months ended September 30, 2012 includes the results of Southeast Texas, a transfer of net assets between entities under common control that was accounted for as if the transfer occurred at the beginning of the period, and prior years are retrospectively adjusted to furnish comparative information similar to the pooling method.

[3] The financial information for the nine months ended September 30, 2011 includes the results of Southeast Texas, a transfer of net assets between entities under common control that was accounted for as if the transfer occurred at the beginning of the period, and prior years are retrospectively adjusted to furnish comparative information similar to the pooling method.

[4] Gross margin consists of total operating revenues, including commodity derivative activity, less purchases of natural gas, propane, NGLs and condensate. Gross margin is viewed as a non-GAAP measure under the rules of the SEC, but is included as a supplemental disclosure because it is a primary performance measure used by management as it represents the results of product sales versus product purchases. As an indicator of our operating performance, gross margin should not be considered an alternative to, or more meaningful than, net income or cash flow as determined in accordance with GAAP. Our gross margin may not be comparable to a similarly titled measure of another company because other entities may not calculate gross margin in the same manner.

[5] For the three and nine months ended September 30, 2011, income tax expense relates primarily to the Texas margin tax and the Michigan business tax. The Michigan business tax was repealed in 2012; accordingly, income tax expense for the three and nine months ended September 30, 2012 relates primarily to the Texas margin tax.

[6] Non-cash derivative mark-to-market is included in segment gross margin, along with cash settlements for our derivative contracts.

[7] The financial information during the nine months ended September 30, 2011 includes the results of Southeast Texas, a transfer of net assets between entities under common control that was accounted for as if the transfer occurred at the beginning of the period, and prior years are retrospectively adjusted to furnish comparative information similar to the pooling method.

[8] The financial information as of December 31, 2011 includes the results of Southeast Texas, a transfer of net assets between entities under common control that was accounted for as if the transfer occurred at the beginning of the period, and prior years are retrospectively adjusted to furnish comparative information similar to the pooling method.

[9] The segment information for the three and nine months ended September 30, 2012 and 2011, and as of December 31, 2011, include the results of Southeast Texas, a transfer of net assets between entities under common control that was accounted for as if the transfer occurred at the beginning of the period, and prior years are retrospectively adjusted to furnish comparative information similar to the pooling method.

[10] Other long-term assets not allocable to segments consist of unrealized gains on derivative instruments, corporate leasehold improvements and other long-term assets.

[11] Represents intersegment revenues consisting of sales of NGLs by Marysville in our NGL Logistics business to our Wholesale Propane business.