Leases - Maturity of our operating lease liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
2019 (remaining six months of 2019)	$ 56,386
2019			$ 68,246
2020	12,868		12,428
2021	10,029		9,478
2022	6,988		6,513
2023	5,838		5,359
2024	3,691
Thereafter	38,506
Thereafter			42,236
Total lease commitments	134,306		144,260
Less imputed interest	20,705
Total lease liabilities	113,601	$ 119,000
Purchased Power Lease Contracts
Lessee, Lease, Description [Line Items]
2019 (remaining six months of 2019)	49,051
2019			54,499
2020	0		0
2021	0		0
2022	0		0
2023	0		0
2024	0
Thereafter	0
Thereafter			0
Total lease commitments	49,051		54,499
Less imputed interest	294
Total lease liabilities	48,757
Land, Property & Equipment Leases
Lessee, Lease, Description [Line Items]
2019 (remaining six months of 2019)	7,335
2019			13,747
2020	12,868		12,428
2021	10,029		9,478
2022	6,988		6,513
2023	5,838		5,359
2024	3,691
Thereafter	38,506
Thereafter			42,236
Total lease commitments	85,255		$ 89,761
Less imputed interest	20,411
Total lease liabilities	$ 64,844