Derivative Instruments - Narrative (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2016 USD ($) Bcf MBbls	Dec. 31, 2015 USD ($) Bcf
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Deferred net gains in accumulated other comprehensive income (loss), net of tax	$ 36,400,000	$ 64,800,000
Net gains on derivative commodity instruments	$ 28,900,000
Derivative Instruments, Gain (Loss) [Line Items]
Maximum additional collateral as percentage of derivative liability	100.00%
Aggregate fair value of derivative instruments with credit-risk related contingencies	$ 42,000,000
Collateral posted	$ 0
Propane swap
Derivative Instruments, Gain (Loss) [Line Items]
Volume of derivative instruments (in Bcf, Mbbls) | MBbls	548
Cash flow hedging | Commodity derivatives
Derivative Instruments, Gain (Loss) [Line Items]
Volume of derivative instruments (in Bcf, Mbbls) | Bcf	665	676