Leases - Lease costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Operating Leased Assets [Line Items]
Operating lease cost	$ 56,317	$ 25,676	$ 82,842	$ 48,502
Variable lease cost	40,890	37,100	102,782	96,279
Short-term lease cost	1,038	812	2,824	3,477
Total lease cost	98,245	63,588	188,448	148,258
Purchased Power Lease Contracts
Operating Leased Assets [Line Items]
Operating lease cost	51,662	21,095	68,883	35,159
Variable lease cost	40,658	36,917	102,052	95,736
Short-term lease cost	0	0	0	0
Total lease cost	92,320	58,012	170,935	130,895
Land, Property & Equipment Leases
Operating Leased Assets [Line Items]
Operating lease cost	4,655	4,581	13,959	13,343
Variable lease cost	232	183	730	543
Short-term lease cost	1,038	812	2,824	3,477
Total lease cost	$ 5,925	$ 5,576	$ 17,513	$ 17,363