Fair Value Measurements - Fair Value of Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets
Nuclear decommissioning trust	$ 871,000	$ 779,586
Total assets	1,036	11,076
Fair value measurement on a recurring basis
Assets
Cash equivalents	10,630
Coal reclamation escrow account	32,542	14,521
Coal reclamation escrow account, other	525
Other	(4,737)	(35,103)
Derivative assets	1,982	19,695
Other	417,499	354,056
Nuclear decommissioning trust	871,000	779,586
Other	413,287	318,953
Total assets	916,154	813,802
Liabilities
Other	1,516	31,049
Derivative Liability	(96,422)	(73,074)
Fair value measurement on a recurring basis | Cash and cash equivalents
Assets
Other	109	795
Nuclear decommissioning trust	7,333	795
Fair value measurement on a recurring basis | US commingled equity funds
Assets
Other	417,390	353,261
Nuclear decommissioning trust	417,390	353,261
Fair value measurement on a recurring basis | U.S. Treasury
Assets
Nuclear decommissioning trust	127,662	95,441
Fair value measurement on a recurring basis | Corporate
Assets
Nuclear decommissioning trust	114,007	111,623
Fair value measurement on a recurring basis | Mortgage-backed securities
Assets
Nuclear decommissioning trust	111,874	115,337
Fair value measurement on a recurring basis | Municipal bonds
Assets
Nuclear decommissioning trust	79,049	80,997
Fair value measurement on a recurring basis | Other
Assets
Nuclear decommissioning trust	13,685	22,132
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Cash equivalents	10,630
Coal reclamation escrow account	455	14,521
Decommissioning fund investments, gross fair value	134,886	95,441
Gross assets, fair value disclosure	145,971	109,962
Liabilities
Gross derivative liability	0	0
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Assets
Decommissioning fund investments, gross fair value		0
Nuclear decommissioning trust	7,224
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury
Assets
Decommissioning fund investments, gross fair value	127,662	95,441
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets
Gross derivative assets	5,683	43,722
Coal reclamation escrow account	31,562
Decommissioning fund investments, gross fair value	318,615	330,089
Gross assets, fair value disclosure	355,860	373,811
Liabilities
Gross derivative liability	(78,646)	(45,641)
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Corporate
Assets
Decommissioning fund investments, gross fair value	114,007	111,623
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Decommissioning fund investments, gross fair value	111,874	115,337
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Municipal bonds
Assets
Decommissioning fund investments, gross fair value	79,049	80,997
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Other
Assets
Decommissioning fund investments, gross fair value	13,685	22,132
Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3)
Assets
Gross derivative assets	1,036	11,076
Gross assets, fair value disclosure	1,036	11,076
Liabilities
Gross derivative liability	$ (19,292)	$ (58,482)