Acquisitions and Divestitures - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended			12 Months Ended
	Jul. 31, 2014	Mar. 31, 2016	Sep. 30, 2014	Mar. 31, 2014	Dec. 31, 2014
Eagle Ford Shale, Lavaca County
Divestitures [Line Items]
Payments to Acquire Businesses, Gross					$ 45.6
Payments to Acquire Oil and Gas Property			$ 34.9
Business Combination, Consideration Transferred, Liabilities Incurred					$ 10.7
Contractual Obligation		$ 1.9
South Texas Oil Gathering Sys. Development Rights
Divestitures [Line Items]
Sale Leaseback Transaction, Deferred Gain, Gross	$ 75.7
Period over which deferred gain is being recognized	25 years
Balance of deferred gain recognized in period		0.1
Deferred gain on sale of assets, current component		3.0
Deferred gain on sale of assets, noncurrent component		72.6
South Texas Natural Gas Gathering Assets
Divestitures [Line Items]
Gain on Sale of Oil and Gas Property				$ 10.6
Period over which deferred gain is being recognized				25 years
Balance of deferred gain recognized in period		0.1
Deferred gain on sale of assets, current component		0.4
Deferred gain on sale of assets, noncurrent component		$ 9.3