Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets
Commodity contracts, assets	$ 740	$ 515
Commodity contracts, liabilities	(2,790)	(69)
Nuclear decommissioning trust	1,024,033	1,010,775
Nuclear decommissioning trust, other	496,121	521,245
Other special use funds	253,332	245,095
Other special use funds, other	526	474
Total assets	1,278,105	1,256,385
Total assets, other	493,857	521,650
Liabilities
Total, other	1,605	711
Amount reported on balance sheet	(70,986)	(72,132)
Equity securities
Assets
Nuclear decommissioning trust	9,855	13,273
Nuclear decommissioning trust, other	(6,130)	2,401
Other special use funds	9,481	7,616
Other special use funds, other	526	474
U.S. commingled equity funds
Assets
Nuclear decommissioning trust	502,251	518,844
U.S. Treasury debt
Assets
Nuclear decommissioning trust	143,673	160,607
Other special use funds	229,858	232,848
Corporate debt
Assets
Nuclear decommissioning trust	149,672	115,869
Mortgage-backed securities
Assets
Nuclear decommissioning trust	99,407	118,795
Municipal bonds
Assets
Nuclear decommissioning trust	106,971	73,040
Other special use funds	13,993	4,631
Other fixed income
Assets
Nuclear decommissioning trust	12,204	10,347
Level 1
Assets
Commodity contracts, assets	0	0
Nuclear decommissioning trust	159,658	171,479
Other special use funds	238,813	239,990
Total assets	398,471	411,469
Liabilities
Gross derivative liability	0	0
Level 1 | Equity securities
Assets
Nuclear decommissioning trust	15,985	10,872
Other special use funds	8,955	7,142
Level 1 | U.S. commingled equity funds
Assets
Nuclear decommissioning trust	0	0
Level 1 | U.S. Treasury debt
Assets
Nuclear decommissioning trust	143,673	160,607
Other special use funds	229,858	232,848
Level 1 | Corporate debt
Assets
Nuclear decommissioning trust	0	0
Level 1 | Mortgage-backed securities
Assets
Nuclear decommissioning trust	0	0
Level 1 | Municipal bonds
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Level 1 | Other fixed income
Assets
Nuclear decommissioning trust	0	0
Level 2
Assets
Commodity contracts, assets	3,165	551
Nuclear decommissioning trust	368,254	318,051
Other special use funds	13,993	4,631
Total assets	385,412	323,233
Liabilities
Gross derivative liability	(61,974)	(67,992)
Level 2 | Equity securities
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Level 2 | U.S. commingled equity funds
Assets
Nuclear decommissioning trust	0	0
Level 2 | U.S. Treasury debt
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Level 2 | Corporate debt
Assets
Nuclear decommissioning trust	149,672	115,869
Level 2 | Mortgage-backed securities
Assets
Nuclear decommissioning trust	99,407	118,795
Level 2 | Municipal bonds
Assets
Nuclear decommissioning trust	106,971	73,040
Other special use funds	13,993	4,631
Level 2 | Other fixed income
Assets
Nuclear decommissioning trust	12,204	10,347
Level 3
Assets
Commodity contracts, assets	365	33
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Total assets	365	33
Liabilities
Gross derivative liability	(10,617)	(3,429)
Level 3 | Equity securities
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Level 3 | U.S. commingled equity funds
Assets
Nuclear decommissioning trust	0	0
Level 3 | U.S. Treasury debt
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Level 3 | Corporate debt
Assets
Nuclear decommissioning trust	0	0
Level 3 | Mortgage-backed securities
Assets
Nuclear decommissioning trust	0	0
Level 3 | Municipal bonds
Assets
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Level 3 | Other fixed income
Assets
Nuclear decommissioning trust	0	0
Fair Value Measured at Net Asset Value Per Share | U.S. commingled equity funds
Assets
Nuclear decommissioning trust	$ 502,251	$ 518,844