Leases - Lease costs (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Operating Leased Assets [Line Items]
Operating lease cost	$ 4,348
Variable lease cost	17,290
Total lease cost	21,638
Purchased Power Lease Contracts [Member]
Operating Leased Assets [Line Items]
Operating lease cost	0
Variable lease cost	17,290
Total lease cost	17,290
Land, Property and Equipment Leases [Member]
Operating Leased Assets [Line Items]
Operating lease cost	4,348
Variable lease cost	0
Total lease cost	$ 4,348