Summary of Significant Accounting Policies - Alternative Fuels Tax Credit (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) $ / gal	Dec. 31, 2018 USD ($) $ / gal	Dec. 31, 2017 USD ($) $ / gal
Summary of Significant Accounting Policies
Federal fuel tax credit - CNG (in dollars per gasoline gallon equivalent) | $ / gal	0.50	0.50	0.50
Federal fuel tax credit - LNG (in dollars per liquid gallon) | $ / gal	0.50	0.50	0.50
VETC credits recognized as revenue | $	$ 47.1	$ 25.2	$ 0.0
VETC credits recognized as revenue from prior years | $		$ 1.5