Leases - Maturity of our operating lease liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
2019 (remaining three months of 2019)	$ 16,977
2019			$ 68,246
2020	14,083		12,428
2021	11,244		9,478
2022	7,727		6,513
2023	6,101		5,359
2024	3,915
Thereafter	38,697
Thereafter			42,236
Total lease commitments	98,744		144,260
Less imputed interest	20,051
Total lease liabilities	78,693	$ 119,000
Purchased Power Lease Contracts
Lessee, Lease, Description [Line Items]
2019 (remaining three months of 2019)	13,625
2019			54,499
2020	0		0
2021	0		0
2022	0		0
2023	0		0
2024	0
Thereafter	0
Thereafter			0
Total lease commitments	13,625		54,499
Less imputed interest	19
Total lease liabilities	13,606
Land, Property & Equipment Leases
Lessee, Lease, Description [Line Items]
2019 (remaining three months of 2019)	3,352
2019			13,747
2020	14,083		12,428
2021	11,244		9,478
2022	7,727		6,513
2023	6,101		5,359
2024	3,915
Thereafter	38,697
Thereafter			42,236
Total lease commitments	85,119		$ 89,761
Less imputed interest	20,032
Total lease liabilities	$ 65,087