Fair Value Measurements - Quantitative Information About Level 3 Fair Value Measurements (Details) $ in Millions		9 Months Ended
		Sep. 30, 2018 USD ($) $ / MMBTU	Dec. 31, 2017 USD ($)
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Derivative assets		$ 6.9	$ 2.4
Fair Value, Measurements, Recurring | Commodity
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Derivative assets		6.9	2.4
Level 3 | Fair Value, Measurements, Recurring
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Derivative assets		6.9
Level 3 | Fair Value, Measurements, Recurring | Commodity
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Derivative assets		6.9	$ 2.4
Level 3 | Fair Value, Measurements, Recurring | Natural Gas | Commodity | Option model
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Derivative assets	[1]	$ 6.9
Level 3 | Fair Value, Measurements, Recurring | Natural Gas | Commodity | Option model | Minimum | Derivative Financial Instruments, Assets
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value inputs, price | $ / MMBTU	[2]	1.8
Level 3 | Fair Value, Measurements, Recurring | Natural Gas | Commodity | Option model | Maximum | Derivative Financial Instruments, Assets
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value inputs, price | $ / MMBTU	[2]	4.1
Level 3 | Fair Value, Measurements, Recurring | Natural Gas | Commodity | Option model | Weighted Average | Derivative Financial Instruments, Assets
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Fair value inputs, price | $ / MMBTU	[3]	2.7

[1] Includes basis.

[2] Represents market prices beyond defined terms for Level 1 and 2.

[3] Averages weighted by volume.