Acquisitions and Divestitures - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended	12 Months Ended
	Jul. 31, 2014	Sep. 30, 2014	Mar. 31, 2014	Jun. 30, 2016	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				$ 2.0
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				0.9
Deferred gain on sale of assets, current component				3.0
Deferred gain on sale of assets, noncurrent component				71.8
South Texas Natural Gas Gathering Assets
Divestitures [Line Items]
Period over which deferred gain is being recognized			25 years
Balance of deferred gain recognized in period				0.2
Deferred gain on sale of assets, current component				0.4
Gain on Sale of Oil and Gas Property			$ 10.6
Deferred gain on sale of assets, noncurrent component				$ 9.1