Divestitures - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2015	Sep. 30, 2014	Mar. 31, 2014	Jun. 30, 2015
Divestitures [Line Items]
Balance of deferred gain recognized in period				$ 0.2
South Texas Oil Gathering Sys. Development Rights
Divestitures [Line Items]
Proceeds from Sale of Intangible Assets		$ 147.0
Sale Leaseback Transaction, Recognized Gain, Net		63.0
Sale Leaseback Transaction, Deferred Gain, Gross		$ 84.1
Period over which deferred gain is being recognized		25 years
Deferred gain on sale of assets, current component				2.6
Deferred gain on sale of assets, noncurrent component				81.5
Gain on Disposition of Assets		$ 147.1
South Texas Natural Gas Gathering Assets
Divestitures [Line Items]
Proceeds from sale of oil and gas properties before closing adjustments			$ 96.0
Gain on Sale of Oil and Gas Property			67.3
Sale Leaseback Transaction, Recognized Gain, Net			56.7
Sale Leaseback Transaction, Deferred Gain, Gross			$ 10.6
Period over which deferred gain is being recognized			25 years
Deferred gain on sale of assets, current component				0.4
Deferred gain on sale of assets, noncurrent component				$ 9.6
Subsequent Event | East Texas
Divestitures [Line Items]
Proceeds from sale of oil and gas properties before closing adjustments	$ 75.0