| **Leases - Maturity of our operating lease liabilities (Details) - USD ($) $ in Thousands** | Mar. 31, 2019 | Jan. 01, 2019 | Dec. 31, 2018 |
|---------------------------------------------------------------------------------------------|---------------|---------------|---------------|
| **Lessee, Lease, Description [Line Items]**                                                 |          |          |          |
| 2019 (remainder of fiscal year)                                                             | $ 65,038      |          |          |
| 2019                                                                                        |          |          | $ 68,246      |
| 2020                                                                                        | 12,424        |          | 12,428        |
| 2021                                                                                        | 9,585         |          | 9,478         |
| 2022                                                                                        | 6,621         |          | 6,513         |
| 2023                                                                                        | 5,496         |          | 5,359         |
| Thereafter                                                                                  | 41,618        |          | 42,236        |
| Total lease commitments                                                                     | 140,782       |          | 144,260       |
| Less imputed interest                                                                       | 21,643        |          |          |
| Total lease liabilities                                                                     | 119,139       | $ 119,000     |          |
| Purchased Power Lease Contracts [Member]                                                    |          |          |          |
| **Lessee, Lease, Description [Line Items]**                                                 |          |          |          |
| 2019 (remainder of fiscal year)                                                             | 54,499        |          |          |
| 2019                                                                                        |          |          | 54,499        |
| 2020                                                                                        | 0             |          | 0             |
| 2021                                                                                        | 0             |          | 0             |
| 2022                                                                                        | 0             |          | 0             |
| 2023                                                                                        | 0             |          | 0             |
| Thereafter                                                                                  | 0             |          | 0             |
| Total lease commitments                                                                     | 54,499        |          | 54,499        |
| Less imputed interest                                                                       | 814           |          |          |
| Total lease liabilities                                                                     | 53,685        |          |          |
| Land, Property and Equipment Leases [Member]                                                |          |          |          |
| **Lessee, Lease, Description [Line Items]**                                                 |          |          |          |
| 2019 (remainder of fiscal year)                                                             | 10,539        |          |          |
| 2019                                                                                        |          |          | 13,747        |
| 2020                                                                                        | 12,424        |          | 12,428        |
| 2021                                                                                        | 9,585         |          | 9,478         |
| 2022                                                                                        | 6,621         |          | 6,513         |
| 2023                                                                                        | 5,496         |          | 5,359         |
| Thereafter                                                                                  | 41,618        |          | 42,236        |
| Total lease commitments                                                                     | 86,283        |          | $ 89,761      |
| Less imputed interest                                                                       | 20,829        |          |          |
| Total lease liabilities                                                                     | $ 65,454      |          |          |

| X                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of lessee's undiscounted obligation for lease payments for operating lease.
+ References
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| - Definition
Amount of lessee's undiscounted obligation for lease payments for operating lease, due after fifth fiscal year following latest fiscal year.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of lessee's undiscounted obligation for lease payments for operating lease, due in next fiscal year following latest fiscal year.
+ References
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| - Definition
Amount of lessee's undiscounted obligation for lease payments for operating lease, due in fifth fiscal year following latest fiscal year.
+ References
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| - Definition
Amount of lessee's undiscounted obligation for lease payments for operating lease, due in fourth fiscal year following latest fiscal year.
+ References
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Amount of lessee's undiscounted obligation for lease payments for operating lease, due in third fiscal year following latest fiscal year.
+ References
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Amount of lessee's undiscounted obligation for lease payments for operating lease, due in second fiscal year following latest fiscal year.
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of lessee's undiscounted obligation for lease payments for operating lease having an initial or remaining lease term in excess of one year due in remainder of fiscal year following latest fiscal year.
+ References
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of lessee's undiscounted obligation for lease payments in excess of discounted obligation for lease payments for operating lease.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
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| - Definition
Present value of lessee's discounted obligation for lease payments from operating lease.
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