Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets
Total	$ 805,048	$ 779,586
Total assets	9,416	11,076
Recurring
Assets
Coal reclamation trust - cash equivalents (b)	14,801	14,521
Derivative instruments, other	(25,625)	(35,103)
Derivative assets	4,222	19,695
Nuclear decommissioning trust, other	375,031	354,056
Total	805,048	779,586
Total, other	349,406	318,953
Total assets	824,071	813,802
Liabilities
Total, other	21,583	31,049
Amount Reported on Balance Sheet	(105,145)	(73,074)
Recurring | US commingled equity funds
Assets
Nuclear decommissioning trust, other	374,695	353,261
Total	374,695	353,261
Recurring | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust, other	336	795
Total	336	795
Recurring | U.S. Treasury
Assets
Total	94,709	95,441
Recurring | Corporate debt
Assets
Total	115,329	111,623
Recurring | Mortgage-backed securities
Assets
Total	115,332	115,337
Recurring | Municipality bonds
Assets
Total	81,932	80,997
Recurring | Other
Assets
Total	22,715	22,132
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Coal reclamation trust - cash equivalents (b)	14,801	14,521
Decommissioning fund investments, gross fair value	94,709	95,441
Gross assets, fair value disclosure	109,510	109,962
Liabilities
Gross derivative liability	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalent funds
Assets
Decommissioning fund investments, gross fair value	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury
Assets
Decommissioning fund investments, gross fair value	94,709	95,441
Recurring | Significant Other Observable Inputs (Level 2)
Assets
Gross derivative assets	20,431	43,722
Decommissioning fund investments, gross fair value	335,308	330,089
Gross assets, fair value disclosure	355,739	373,811
Liabilities
Gross derivative liability	(75,627)	(45,641)
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Decommissioning fund investments, gross fair value	115,329	111,623
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Decommissioning fund investments, gross fair value	115,332	115,337
Recurring | Significant Other Observable Inputs (Level 2) | Municipality bonds
Assets
Decommissioning fund investments, gross fair value	81,932	80,997
Recurring | Significant Other Observable Inputs (Level 2) | Other
Assets
Decommissioning fund investments, gross fair value	22,715	22,132
Recurring | Significant Unobservable Inputs (a) (Level 3)
Assets
Gross derivative assets	9,416	11,076
Gross assets, fair value disclosure	9,416	11,076
Liabilities
Gross derivative liability	$ (51,101)	$ (58,482)