Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets
Cash equivalents		$ 1,200
Commodity contracts, assets	$ 817	1,113
Commodity contracts, liabilities	(959)	(2,029)
Nuclear decommissioning trust	967,673	851,134
Nuclear decommissioning trust, other	475,371	398,953
Other special use funds	243,982	236,101
Other special use funds, other	1,418	593
Total assets	1,212,472	1,089,548
Total assets, other	475,830	397,517
Liabilities
Total, other	80	875
Amount reported on balance sheet	(71,654)	(60,037)
Equity securities
Assets
Nuclear decommissioning trust	7,452	7,351
Nuclear decommissioning trust, other	(1,322)	2,148
Other special use funds	3,400	45,723
Other special use funds, other	1,418	593
U.S. commingled equity funds
Assets
Nuclear decommissioning trust	476,693	396,805
U.S. Treasury debt
Assets
Nuclear decommissioning trust	162,092	148,173
Other special use funds	232,165	173,310
Corporate debt
Assets
Nuclear decommissioning trust	124,026	96,656
Mortgage-backed securities
Assets
Nuclear decommissioning trust	112,704	113,115
Municipal bonds
Assets
Nuclear decommissioning trust	74,202	79,073
Other special use funds	8,417	17,068
Other fixed income
Assets
Nuclear decommissioning trust	10,504	9,961
Level 1
Assets
Cash equivalents		1,200
Commodity contracts, assets	0	0
Nuclear decommissioning trust	170,866	153,376
Other special use funds	234,147	218,440
Total assets	405,013	373,016
Liabilities
Gross derivative liability	0	0
Level 1 | Equity securities
Assets
Nuclear decommissioning trust	8,774	5,203
Other special use funds	1,982	45,130
Level 1 | U.S. commingled equity funds
Assets
Nuclear decommissioning trust	0	0
Level 1 | U.S. Treasury debt
Assets
Nuclear decommissioning trust	162,092	148,173
Other special use funds	232,165	173,310
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	1,388	3,140
Nuclear decommissioning trust	321,436	298,805
Other special use funds	8,417	17,068
Total assets	331,241	319,013
Liabilities
Gross derivative liability	(69,752)	(52,696)
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	124,026	96,656
Level 2 | Mortgage-backed securities
Assets
Nuclear decommissioning trust	112,704	113,115
Level 2 | Municipal bonds
Assets
Nuclear decommissioning trust	74,202	79,073
Other special use funds	8,417	17,068
Level 2 | Other fixed income
Assets
Nuclear decommissioning trust	10,504	9,961
Level 3
Assets
Cash equivalents		0
Commodity contracts, assets	388	2
Nuclear decommissioning trust	0	0
Other special use funds	0	0
Total assets	388	2
Liabilities
Gross derivative liability	(1,982)	(8,216)
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	$ 476,693	$ 396,805