Leases - Lease costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Operating Leased Assets [Line Items]
Operating lease cost	$ 21,872	$ 18,477	$ 26,525	$ 22,825
Variable lease cost	41,076	41,889	61,892	59,180
Short-term lease cost	996	1,812	1,786	2,665
Total lease cost	63,944	62,178	90,203	84,670
Purchased Power Lease Contracts
Operating Leased Assets [Line Items]
Operating lease cost	17,221	14,063	17,221	14,063
Variable lease cost	40,821	41,529	61,394	58,820
Short-term lease cost	0	0	0	0
Total lease cost	58,042	55,592	78,615	72,883
Land, Property & Equipment Leases
Operating Leased Assets [Line Items]
Operating lease cost	4,651	4,414	9,304	8,762
Variable lease cost	255	360	498	360
Short-term lease cost	996	1,812	1,786	2,665
Total lease cost	$ 5,902	$ 6,586	$ 11,588	$ 11,787