Leases - Balance Sheet Impact (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Right-of-Use Assets—Finance leases
Right-of-use assets	$ 302,732	$ 302,732
Less: Accumulated provision for depreciation	(258,821)	(257,504)
Total finance lease assets	43,911	45,228
Lease liabilities—Finance leases
Obligations under finance leases	75,649	75,649
Long-term debt and finance leases due within one year	6,081	6,081
Total finance lease liabilities	81,730	81,730
Right-of-Use Assets—Operating leases
Electric plant in service	4,033	3,237
Total operating lease assets	4,033	3,237
Lease liabilities—Operating leases
Capitalization—Other	3,056	2,293
Other current liabilities	991	1,252
Total operating lease liabilities	$ 4,047	$ 3,545