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