Schedule of Preliminary Purchase Price Allocation (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Jul. 02, 2012	Dec. 31, 2011		Sep. 30, 2012 Crossroads System [Member]	Jul. 03, 2012 Crossroads System [Member]
Schedule Of Business Acquisitions Purchase Price Allocation [Line Items]
Aggregate consideration					$ 63.0
Accounts receivable					4.2
Property, plant and equipment					63.1
Investments in unconsolidated affiliates	229.0		107.1	[1]	6.1
Other current liabilities					(4.1)
Other long-term liabilities					(6.3)
Total preliminary purchase price allocation		$ 200.0			$ 63.0	$ 63.0

[1] 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.