Summary of Significant Accounting Policies - Additional Information 2 (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) Customer Counterparty	Dec. 31, 2014 USD ($) Customer	Dec. 31, 2013 Customer
Summary Of Significant Accounting Policies [Line Items]
Allowance for doubtful accounts	$ 5.0	$ 2.7
Number of customers accounted more than 10% of total oil and gas revenues | Customer	1	4	4
Net derivative asset	$ 11.5
Number of counterparties | Counterparty	4
Total Number of Derivative Counterparties | Counterparty	17
Accounting Standards Update 2015-04 | Subordinated Debt
Summary Of Significant Accounting Policies [Line Items]
Prior period reclassification		$ 32.4
Accounting Standards Update 2015-04 | Bank Credit Facility
Summary Of Significant Accounting Policies [Line Items]
Prior period reclassification		9.8
Accounting Standards Update 2015-11
Summary Of Significant Accounting Policies [Line Items]
Prior period reclassification		$ 115.6