Leases - Cottonwood Sale Leaseback Agreement (Details) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)
Lessor, Lease, Description [Line Items]
Depreciation expense	$ 6.2	$ 9.5
Cottonwood Energy
Lessor, Lease, Description [Line Items]
Fixed lease payments per year	$ 40.0	$ 40.0