Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Assets
Total	$ 822,244	$ 779,586
Total assets	6,440	11,076
Municipal bonds
Assets
Coal reclamation escrow amount	14,839
Recurring
Assets
Coal reclamation escrow amount		14,521
Derivative instruments, other	(16,882)	(35,103)
Derivative assets	362	19,695
Nuclear decommissioning trust, other	386,832	354,056
Total	822,244	779,586
Total, other	369,950	318,953
Total assets	837,445	813,802
Liabilities
Total, other	17,598	31,049
Amount Reported on Balance Sheet	(95,199)	(73,074)
Recurring | US commingled equity funds
Assets
Nuclear decommissioning trust, other	384,999	353,261
Total	384,999	353,261
Recurring | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust, other	1,833	795
Total	1,833	795
Recurring | U.S. Treasury
Assets
Total	101,337	95,441
Recurring | Corporate debt
Assets
Total	120,288	111,623
Recurring | Mortgage-backed securities
Assets
Total	113,950	115,337
Recurring | Municipal bonds
Assets
Total	80,659	80,997
Recurring | Other
Assets
Total	19,178	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	101,337	95,441
Gross assets, fair value disclosure	101,337	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	101,337	95,441
Recurring | Significant Other Observable Inputs (Level 2)
Assets
Gross derivative assets	10,804	43,722
Decommissioning fund investments, gross fair value	334,075	330,089
Gross assets, fair value disclosure	359,718	373,811
Liabilities
Gross derivative liability	(70,112)	(45,641)
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Decommissioning fund investments, gross fair value	120,288	111,623
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Decommissioning fund investments, gross fair value	113,950	115,337
Recurring | Significant Other Observable Inputs (Level 2) | Municipal bonds
Assets
Coal reclamation escrow amount	14,839
Decommissioning fund investments, gross fair value	80,659	80,997
Recurring | Significant Other Observable Inputs (Level 2) | Other
Assets
Decommissioning fund investments, gross fair value	19,178	22,132
Recurring | Significant Unobservable Inputs (Level 3)
Assets
Gross derivative assets	6,440	11,076
Gross assets, fair value disclosure	6,440	11,076
Liabilities
Gross derivative liability	$ (42,685)	$ (58,482)