Fair Value Measurements - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Assets
Nuclear decommissioning trust	$ 779,753	$ 735,196
Total assets	14,835	30,364
Recurring
Assets
Cash equivalents	40,075
Coal reclamation trust - Cash equivalents (b)	14,749
Derivative instruments, other	(22,798)	(25,345)
Derivative assets	6,235	28,011
Nuclear decommissioning trust, other	345,468	314,622
Nuclear decommissioning trust	779,753	735,196
Total, other	322,670	289,277
Total assets	840,812	763,207
Liabilities
Total, other	18,551	39,698
Derivative liability	(103,518)	(167,689)
Recurring | US commingled equity funds
Assets
Nuclear decommissioning trust, other	340,436	314,957
Nuclear decommissioning trust	340,436	314,957
Recurring | Cash and cash equivalent funds
Assets
Nuclear decommissioning trust, other	5,032	(335)
Nuclear decommissioning trust	5,032	11,925
Recurring | U.S. Treasury
Assets
Nuclear decommissioning trust	101,394	117,245
Recurring | Corporate debt
Assets
Nuclear decommissioning trust	112,923	96,243
Recurring | Mortgage-backed securities
Assets
Nuclear decommissioning trust	112,485	99,065
Recurring | Municipality bonds
Assets
Nuclear decommissioning trust	82,371	72,206
Recurring | Other
Assets
Nuclear decommissioning trust	25,112	23,555
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Cash equivalents	40,075
Coal reclamation trust - Cash equivalents (b)	14,749
Decommissioning fund investments, gross fair value	101,394	129,505
Gross assets, fair value disclosure	156,218	129,505
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalent funds
Assets
Decommissioning fund investments, gross fair value	0	12,260
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury
Assets
Decommissioning fund investments, gross fair value	101,394	117,245
Recurring | Significant Other Observable Inputs (Level 2)
Assets
Gross derivative assets	14,198	22,992
Decommissioning fund investments, gross fair value	332,891	291,069
Gross assets, fair value disclosure	347,089	314,061
Liabilities
Gross derivative liability	(67,692)	(144,044)
Recurring | Significant Other Observable Inputs (Level 2) | Corporate debt
Assets
Decommissioning fund investments, gross fair value	112,923	96,243
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Decommissioning fund investments, gross fair value	112,485	99,065
Recurring | Significant Other Observable Inputs (Level 2) | Municipality bonds
Assets
Decommissioning fund investments, gross fair value	82,371	72,206
Recurring | Significant Other Observable Inputs (Level 2) | Other
Assets
Decommissioning fund investments, gross fair value	25,112	23,555
Recurring | Significant Unobservable Inputs (a) (Level 3)
Assets
Gross derivative assets	14,835	30,364
Gross assets, fair value disclosure	14,835	30,364
Liabilities
Gross derivative liability	$ (54,377)	$ (63,343)