Fair Value Measurements - Quantitative Information About Level 3 Fair Value Measurements (Details) $ in Millions		3 Months Ended
		Mar. 31, 2018 USD ($) $ / MMBTU	Dec. 31, 2017 USD ($)
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Derivative assets		$ 1.4	$ 2.4
Fair Value, Measurements, Recurring | Commodity
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Derivative assets		1.4	2.4
Level 3 | Fair Value, Measurements, Recurring
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Derivative assets		1.4
Level 3 | Fair Value, Measurements, Recurring | Commodity
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Derivative assets		1.4	$ 2.4
Level 3 | Fair Value, Measurements, Recurring | Natural Gas | Commodity | Option model
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Derivative assets	[1]	$ 1.4
Level 3 | Fair Value, Measurements, Recurring | Natural Gas | Commodity | Option model | Minimum | Derivative Financial Instruments, Assets
Fair Value Inputs, Liabilities, Quantitative Information [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 Inputs, Liabilities, Quantitative Information [Line Items]
Fair value inputs, price | $ / MMBTU	[2]	3.0
Level 3 | Fair Value, Measurements, Recurring | Natural Gas | Commodity | Option model | Weighted Average | Derivative Financial Instruments, Assets
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair value inputs, price | $ / MMBTU	[3]	2.5

[1] Includes basis.

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

[3] Averages weighted by volume.