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Leases - Future minimum operating lease (Details)
$ in Thousands
Dec. 31, 2018
USD ($)
Lessee, Lease, Description [Line Items]  
2019 $ 68,246
2020 12,428
2021 9,478
2022 6,513
2023 5,359
Thereafter 42,236
Total future lease commitments 144,260
Purchased Power Lease Contracts  
Lessee, Lease, Description [Line Items]  
2019 54,499
2020 0
2021 0
2022 0
2023 0
Thereafter 0
Total future lease commitments 54,499
Land, Property & Equipment Leases  
Lessee, Lease, Description [Line Items]  
2019 13,747
2020 12,428
2021 9,478
2022 6,513
2023 5,359
Thereafter 42,236
Total future lease commitments $ 89,761