Leases - Lease costs (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019
Operating Leased Assets [Line Items]
Operating lease cost	$ 25,676	$ 48,502
Variable lease cost	37,100	96,279
Short-term lease cost	812	3,477
Total lease cost	63,588	148,258
Purchased Power Lease Contracts
Operating Leased Assets [Line Items]
Operating lease cost	21,095	35,159
Variable lease cost	36,917	95,736
Short-term lease cost	0	0
Total lease cost	58,012	130,895
Land, Property & Equipment Leases
Operating Leased Assets [Line Items]
Operating lease cost	4,581	13,343
Variable lease cost	183	543
Short-term lease cost	812	3,477
Total lease cost	$ 5,576	$ 17,363