Leases - Lease costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Operating Leased Assets [Line Items]
Operating lease cost	$ 87,376	$ 60,228
Variable lease cost	122,331	114,015
Short-term lease cost	3,804	4,385
Total lease cost	213,511	178,628
Purchased Power Lease Contracts
Operating Leased Assets [Line Items]
Operating lease cost	68,883	42,190
Variable lease cost	121,359	113,233
Short-term lease cost	0	0
Total lease cost	190,242	155,423
Land, Property & Equipment Leases
Operating Leased Assets [Line Items]
Operating lease cost	18,493	18,038
Variable lease cost	972	782
Short-term lease cost	3,804	4,385
Total lease cost	$ 23,269	$ 23,205