Fair Value Measurements - Fair Value of Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets
Cash equivalents	$ 1,200	$ 10,630
Other	(2,029)	(4,737)
Derivative assets	1,113	1,982
Other	398,953	417,499
Nuclear decommissioning trust	851,134	871,000
Other special use funds, gross fair value	236,101	32,542
Other special use funds, other	593	525
Other	397,517	413,287
Total assets	1,089,548	916,154
Liabilities
Other	875	1,516
Derivative Liability	(60,037)	(96,422)
Cash and cash equivalents
Assets
Other		109
Nuclear decommissioning trust		7,333
Equity securities
Assets
Other	2,148
Nuclear decommissioning trust	7,351
Other special use funds, gross fair value	45,723
Other special use funds, other	593
US commingled equity funds
Assets
Other	396,805	417,390
Nuclear decommissioning trust	396,805	417,390
U.S. Treasury debt
Assets
Nuclear decommissioning trust	148,173	127,662
Other special use funds, gross fair value	173,310
Corporate debt
Assets
Nuclear decommissioning trust	96,656	114,007
Mortgage-backed securities
Assets
Nuclear decommissioning trust	113,115	111,874
Municipal bonds
Assets
Nuclear decommissioning trust	79,073	79,049
Other special use funds, gross fair value	17,068
Other
Assets
Nuclear decommissioning trust	9,961	13,685
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Cash equivalents	1,200	10,630
Decommissioning fund investments, gross fair value	153,376	134,886
Other special use funds, gross fair value	218,440	455
Total assets	373,016	145,971
Liabilities
Gross derivative liability	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Assets
Nuclear decommissioning trust		7,224
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equity securities
Assets
Nuclear decommissioning trust	5,203
Other special use funds, gross fair value	45,130
Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury debt
Assets
Decommissioning fund investments, gross fair value	148,173	127,662
Other special use funds, gross fair value	173,310
Quoted Prices in Active Markets for Identical Assets (Level 1) | Municipal bonds
Assets
Other special use funds, gross fair value	0
Significant Other Observable Inputs (Level 2)
Assets
Gross derivative assets	3,140	5,683
Decommissioning fund investments, gross fair value	298,805	318,615
Other special use funds, gross fair value	17,068	31,562
Total assets	319,013	355,860
Liabilities
Gross derivative liability	(52,696)	(78,646)
Significant Other Observable Inputs (Level 2) | Equity securities
Assets
Other special use funds, gross fair value	0
Significant Other Observable Inputs (Level 2) | U.S. Treasury debt
Assets
Other special use funds, gross fair value	0
Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Decommissioning fund investments, gross fair value	96,656	114,007
Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Decommissioning fund investments, gross fair value	113,115	111,874
Significant Other Observable Inputs (Level 2) | Municipal bonds
Assets
Decommissioning fund investments, gross fair value	79,073	79,049
Other special use funds, gross fair value	17,068
Significant Other Observable Inputs (Level 2) | Other
Assets
Decommissioning fund investments, gross fair value	9,961	13,685
Significant Unobservable Inputs (Level 3)
Assets
Gross derivative assets	2	1,036
Other special use funds, gross fair value	0
Total assets	2	1,036
Liabilities
Gross derivative liability	(8,216)	$ (19,292)
Significant Unobservable Inputs (Level 3) | Equity securities
Assets
Other special use funds, gross fair value	0
Significant Unobservable Inputs (Level 3) | U.S. Treasury debt
Assets
Other special use funds, gross fair value	0
Significant Unobservable Inputs (Level 3) | Municipal bonds
Assets
Other special use funds, gross fair value	$ 0