Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets
Total	$ 841,980	$ 779,586
Total assets	3,472	11,076
Municipal bonds
Assets
Coal reclamation escrow amount	24,973
Recurring
Assets
Coal reclamation escrow amount		14,521
Derivative instruments, other	(9,851)	(35,103)
Derivative assets	2,050	19,695
Nuclear decommissioning trust, other	406,291	354,056
Total	841,980	779,586
Total, other	396,950	318,953
Total assets	876,688	813,802
Liabilities
Total, other	9,247	31,049
Amount Reported on Balance Sheet	(86,244)	(73,074)
Recurring | US commingled equity funds
Assets
Nuclear decommissioning trust, other	401,913	353,261
Total	401,913	353,261
Recurring | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust, other	4,378	795
Total	14,976	795
Recurring | U.S. Treasury
Assets
Total	90,776	95,441
Recurring | Corporate debt
Assets
Total	124,369	111,623
Recurring | Mortgage-backed securities
Assets
Total	116,237	115,337
Recurring | Municipal bonds
Assets
Total	76,412	80,997
Recurring | Other
Assets
Total	17,297	22,132
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Coal reclamation escrow amount		14,521
Decommissioning fund investments, gross fair value	101,374	95,441
Gross assets, fair value disclosure	108,549	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	10,598	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury
Assets
Decommissioning fund investments, gross fair value	90,776	95,441
Recurring | Significant Other Observable Inputs (Level 2)
Assets
Gross derivative assets	8,429	43,722
Decommissioning fund investments, gross fair value	334,315	330,089
Gross assets, fair value disclosure	367,717	373,811
Liabilities
Gross derivative liability	(53,414)	(45,641)
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Decommissioning fund investments, gross fair value	124,369	111,623
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Decommissioning fund investments, gross fair value	116,237	115,337
Recurring | Significant Other Observable Inputs (Level 2) | Municipal bonds
Assets
Coal reclamation escrow amount	24,973
Decommissioning fund investments, gross fair value	76,412	80,997
Recurring | Significant Other Observable Inputs (Level 2) | Other
Assets
Decommissioning fund investments, gross fair value	17,297	22,132
Recurring | Significant Unobservable Inputs (Level 3)
Assets
Gross derivative assets	3,472	11,076
Gross assets, fair value disclosure	3,472	11,076
Liabilities
Gross derivative liability	(42,077)	$ (58,482)
Cash equivalents | Recurring
Assets
Coal reclamation escrow amount	7,685
Coal reclamation escrow amount, other	510
Cash equivalents | Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Coal reclamation escrow amount	$ 7,175