Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Assets
Cash equivalents	$ 146,680
Commodity contracts, assets	17,304	$ 515
Commodity contracts, liabilities	(3,209)	(69)
Nuclear decommissioning trust	1,069,837	1,010,775
Nuclear decommissioning trust, other	531,697	521,245
Other special use funds	268,292	245,095
Other special use funds, other	1,444	474
Total assets	1,502,113	1,256,385
Total assets, other	529,932	521,650
Liabilities
Gross derivative liability, other	1,925
Gross derivative liability, other		711
Amount reported on balance sheet	(13,358)	(72,132)
Equity securities
Assets
Nuclear decommissioning trust	10,389	13,273
Nuclear decommissioning trust, other	(14,148)	2,401
Other special use funds	9,534	7,616
Other special use funds, other	1,444	474
U.S. commingled equity funds
Assets
Nuclear decommissioning trust	545,845	518,844
U.S. Treasury debt
Assets
Nuclear decommissioning trust	153,213	160,607
Other special use funds	245,156	232,848
Corporate debt
Assets
Nuclear decommissioning trust	146,682	115,869
Mortgage-backed securities
Assets
Nuclear decommissioning trust	101,456	118,795
Municipal bonds
Assets
Nuclear decommissioning trust	99,192	73,040
Other special use funds	13,602	4,631
Other fixed income
Assets
Nuclear decommissioning trust	13,060	10,347
Level 1
Assets
Cash equivalents	146,680
Commodity contracts, assets	0	0
Nuclear decommissioning trust	177,750	171,479
Other special use funds	253,246	239,990
Total assets	577,676	411,469
Liabilities
Gross derivative liability	0	0
Level 1 | Equity securities
Assets
Nuclear decommissioning trust	24,537	10,872
Other special use funds	8,090	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	153,213	160,607
Other special use funds	245,156	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
Cash equivalents	0
Commodity contracts, assets	20,206	551
Nuclear decommissioning trust	360,390	318,051
Other special use funds	13,602	4,631
Total assets	394,198	323,233
Liabilities
Gross derivative liability	(10,737)	(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	146,682	115,869
Level 2 | Mortgage-backed securities
Assets
Nuclear decommissioning trust	101,456	118,795
Level 2 | Municipal bonds
Assets
Nuclear decommissioning trust	99,192	73,040
Other special use funds	13,602	4,631
Level 2 | Other fixed income
Assets
Nuclear decommissioning trust	13,060	10,347
Level 3
Assets
Cash equivalents	0
Commodity contracts, assets	307	33
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Total assets	307	33
Liabilities
Gross derivative liability	(4,546)	(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	$ 545,845	$ 518,844