Fair Value Measurements - Fair Value of Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets
Commodity contracts, assets	$ 4,749	$ 515
Commodity contracts, liabilities	(4,271)	(69)
Nuclear decommissioning trust	1,138,435	1,010,775
Nuclear decommissioning trust, other	592,227	521,245
Other special use fund	254,509	245,095
Other special use funds, other	504	474
Total assets	1,397,693	1,256,385
Total assets, other	588,460	521,650
Liabilities
Gross derivative liability, other	2,986
Gross derivative liability, other		(711)
Amount reported on balance sheet	(18,619)	(72,132)
Equity securities
Assets
Nuclear decommissioning trust	11,968	13,273
Nuclear decommissioning trust, other	(17,828)	2,401
Other special use fund	37,841	7,616
Other special use funds, other	504	474
U.S. commingled equity funds
Assets
Nuclear decommissioning trust	610,055	518,844
U.S. Treasury debt
Assets
Nuclear decommissioning trust	164,514	160,607
Other special use fund	203,220	232,848
Corporate debt
Assets
Nuclear decommissioning trust	149,509	115,869
Mortgage-backed securities
Assets
Nuclear decommissioning trust	99,623	118,795
Municipal bonds
Assets
Nuclear decommissioning trust	89,705	73,040
Other special use fund	13,448	4,631
Other fixed income
Assets
Nuclear decommissioning trust	13,061	10,347
Level 1
Assets
Commodity contracts, assets	0	0
Nuclear decommissioning trust	194,310	171,479
Other special use fund	240,557	239,990
Total assets	434,867	411,469
Liabilities
Gross derivative liability	0	0
Level 1 | Equity securities
Assets
Nuclear decommissioning trust	29,796	10,872
Other special use fund	37,337	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	164,514	160,607
Other special use fund	203,220	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 fund	0	0
Level 1 | Other fixed income
Assets
Nuclear decommissioning trust	0	0
Level 2
Assets
Commodity contracts, assets	9,016	551
Nuclear decommissioning trust	351,898	318,051
Other special use fund	13,448	4,631
Total assets	374,362	323,233
Liabilities
Gross derivative liability	(20,498)	(67,992)
Level 2 | Equity securities
Assets
Nuclear decommissioning trust	0	0
Other special use fund	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 fund	0	0
Level 2 | Corporate debt
Assets
Nuclear decommissioning trust	149,509	115,869
Level 2 | Mortgage-backed securities
Assets
Nuclear decommissioning trust	99,623	118,795
Level 2 | Municipal bonds
Assets
Nuclear decommissioning trust	89,705	73,040
Other special use fund	13,448	4,631
Level 2 | Other fixed income
Assets
Nuclear decommissioning trust	13,061	10,347
Level 3
Assets
Commodity contracts, assets	4	33
Nuclear decommissioning trust	0	0
Other special use fund	0	0
Total assets	4	33
Liabilities
Gross derivative liability	(1,107)	(3,429)
Level 3 | Equity securities
Assets
Nuclear decommissioning trust	0	0
Other special use fund	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 fund	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 fund	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	$ 610,055	$ 518,844