Debt (Narrative) (Detail) (USD $)	1 Months Ended		9 Months Ended										9 Months Ended		1 Months Ended					1 Months Ended	9 Months Ended			1 Months Ended	9 Months Ended
	Mar. 13, 2012	Sep. 30, 2010	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012 East Texas System [Member]	Jan. 31, 2012 East Texas System [Member]	Jan. 03, 2012 East Texas System [Member]	Sep. 30, 2012 Southeast Texas [Member]	Mar. 31, 2012 Southeast Texas [Member]	Jan. 31, 2011 Southeast Texas [Member]	Sep. 30, 2012 Numerator [Member] Acquisition 1 [Member]	Sep. 30, 2012 Denominator [Member] Acquisition 1 [Member]	Mar. 31, 2012 Senior Notes, 4.95% Due 2022 [Member]	Sep. 30, 2012 Senior Notes, 4.95% Due 2022 [Member]	Mar. 13, 2012 Senior Notes, 4.95% Due 2022 [Member]	Jan. 03, 2012 Senior Notes, 4.95% Due 2022 [Member]	Mar. 31, 2012 Senior Notes, 4.95% Due 2022 [Member] Southeast Texas [Member]	Sep. 30, 2012 Credit Agreement [Member]	Sep. 30, 2012 Credit Agreement [Member]	Sep. 30, 2012 Credit Agreement [Member] Numerator [Member]	Sep. 30, 2012 Credit Agreement [Member] Denominator [Member]	Jul. 02, 2012 Term Loan Agreement [Member]	Sep. 30, 2012 Term Loan Agreement [Member]	Jan. 03, 2012 Term Loan Agreement [Member]
Line of Credit Facility [Line Items]
Revolving credit facility																				$ 1,000,000,000	$ 1,000,000,000
Line of credit facility, maturity date			Nov. 10, 2016																		Nov. 10, 2016
Letters of credit issued																				1,000,000
Letters of credit outstanding																				1,000,000	1,000,000
Unused capacity under the revolving credit facility																				699,000,000	699,000,000
Unused capacity available for general working capital																				685,300,000	685,300,000
Line of credit facility interest rate description			(1) LIBOR, plus an applicable margin of 1.25% based on our current credit rating; or (2) (a) the base rate which shall be the higher of Wells Fargo Bank N.A.'s prime rate, the Federal Funds rate plus 0.50% or the LIBOR Market Index rate plus 1%, plus (b) an applicable margin of 0.25% based on our current credit rating.
Annual facility fee description			The revolving credit facility incurs an annual facility fee of 0.25% based on our current credit rating.																					LIBOR, plus an applicable margin of 1.375% based on our current credit rating; or (2) (a) the higher of SunTrust Bank’s prime rate, the Federal Funds rate plus 0.50% or the LIBOR Market Index rate plus 1%, plus (b) an applicable margin of 0.25% based on our current credit rating. The Term Loan Agreement requires us to maintain a leverage ratio (the ratio of our consolidated indebtedness to our consolidated EBITDA, in each case as is defined by the Term Loan Agreement) consistent with our Credit Agreement.
Leverage ratio													5.5	1.0								5.0	1.0
Senior notes issued															350,000,000	350,000,000
Senior notes interest rate percentage	4.95%	3.25%													4.95%	4.95%	4.95%	4.95%
Maturity date	Apr. 01, 2022	Oct. 01, 2015													Apr. 01, 2022										Jul. 02, 2014
Proceeds from debt			1,353,400,000	[1]	832,000,000	[2]									345,800,000
Underwriters' fees															4,200,000
First payment date															Oct. 01, 2012
Acquisition of additional interest							49.90%	49.90%	49.90%	66.67%	66.67%	33.33%							66.67%
Term Loan Agreement																								140,000,000
Term Loan period																								2 years
Term Loan payment			On January 2, 2013 and July 2, 2013, one-time payments of 0.125% and 0.20%, respectively, on the outstanding principal amount of the term loan are required.
Line of credit facility amount borrowed																										$ 135,000,000

[1] 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.

[2] 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.