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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)

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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash -URI http://asc.fasb.org/extlink&oid=6506951 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash Equivalents -URI http://asc.fasb.org/extlink&oid=6507016 + Details
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