Fair Value Measurements - Fair Value of Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Assets
Nuclear decommissioning trust	$ 779,586	$ 735,196
Total assets	11,076	30,364
Fair value measurement on a recurring basis
Assets
Coal reclamation trust - Cash equivalents	14,521
Other	(35,103)	(25,345)
Derivative assets	19,695	28,011
Other	354,056	314,622
Nuclear decommissioning trust	779,586	735,196
Other	318,953	289,277
Total assets	813,802	763,207
Liabilities
Other	31,049	39,698
Derivative Liability	(73,074)	(167,689)
Fair value measurement on a recurring basis | US commingled equity funds
Assets
Other	353,261	314,957
Nuclear decommissioning trust	353,261	314,957
Fair value measurement on a recurring basis | Cash and cash equivalents
Assets
Other	795	(335)
Nuclear decommissioning trust	795	11,925
Fair value measurement on a recurring basis | U.S. Treasury
Assets
Nuclear decommissioning trust	95,441	117,245
Fair value measurement on a recurring basis | Corporate
Assets
Nuclear decommissioning trust	111,623	96,243
Fair value measurement on a recurring basis | Mortgage-backed securities
Assets
Nuclear decommissioning trust	115,337	99,065
Fair value measurement on a recurring basis | Municipality bonds
Assets
Nuclear decommissioning trust	80,997	72,206
Fair value measurement on a recurring basis | Other
Assets
Nuclear decommissioning trust	22,132	23,555
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Coal reclamation trust - Cash equivalents	14,521
Decommissioning fund investments, gross fair value	95,441	129,505
Gross assets, fair value disclosure	109,962	129,505
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		12,260
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	95,441	117,245
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets
Gross derivative assets	43,722	22,992
Decommissioning fund investments, gross fair value	330,089	291,069
Gross assets, fair value disclosure	373,811	314,061
Liabilities
Gross derivative liability	(45,641)	(144,044)
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Corporate
Assets
Decommissioning fund investments, gross fair value	111,623	96,243
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Decommissioning fund investments, gross fair value	115,337	99,065
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Municipality bonds
Assets
Decommissioning fund investments, gross fair value	80,997	72,206
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Other
Assets
Decommissioning fund investments, gross fair value	22,132	23,555
Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3)
Assets
Gross derivative assets	11,076	30,364
Gross assets, fair value disclosure	11,076	30,364
Liabilities
Gross derivative liability	$ (58,482)	$ (63,343)