Leases - Balance Sheet Impact (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Right-of-Use Assets - Finance leases
Right-of-use assets	$ 302,732	$ 302,732
Less: Accumulated provision for depreciation	(257,504)	(252,233)
Total finance lease assets	45,228	50,499
Lease liabilities - Finance leases
Obligations under finance leases	75,649
Long-term debt and finance leases due within one year	6,081
Total finance lease liabilities	81,730
Obligations under finance leases		81,730
Long-term debt and finance leases due within one year		5,461
Total finance lease liabilities		$ 87,191
Right-of-Use Assets - Operating leases
Electric plant in service	3,237
Total operating lease assets	3,237
Lease liabilities - Operating leases
Capitalization - Other	2,293
Other current liabilities	1,252
Total operating lease liabilities	$ 3,545