Fair Value Measurements - Fair Value of Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets
Nuclear decommissioning trust	$ 735,196	$ 713,866
Total assets	30,364	32,598
Fair value measurement on a recurring basis
Assets
Other	(25,345)	(21,962)
Derivative assets	28,011	31,405
Other	(335)	(7,245)
Nuclear decommissioning trust	735,196	713,866
Other	(25,680)	(29,207)
Total assets	763,207	745,271
Liabilities
Other	39,698	58,767
Derivative Liability	(167,689)	(110,278)
Fair value measurement on a recurring basis | US commingled equity funds
Assets
Other	0	0
Nuclear decommissioning trust	314,957	309,620
Fair value measurement on a recurring basis | Cash and cash equivalents
Assets
Other	(335)	(7,245)
Nuclear decommissioning trust	11,925	4,208
Fair value measurement on a recurring basis | U.S. Treasury
Assets
Nuclear decommissioning trust	117,245	118,843
Fair value measurement on a recurring basis | Corporate
Assets
Nuclear decommissioning trust	96,243	109,379
Fair value measurement on a recurring basis | Mortgage-backed securities
Assets
Nuclear decommissioning trust	99,065	88,465
Fair value measurement on a recurring basis | Municipality bonds
Assets
Nuclear decommissioning trust	72,206	69,139
Fair value measurement on a recurring basis | Other (a)
Assets
Nuclear decommissioning trust	23,555	14,212
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets
Decommissioning fund investments, gross fair value	129,505	118,843
Gross assets, fair value disclosure	129,505	118,843
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Cash and cash equivalents
Assets
Decommissioning fund investments, gross fair value	12,260
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury
Assets
Decommissioning fund investments, gross fair value	117,245	118,843
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets
Gross derivative assets	22,992	20,769
Decommissioning fund investments, gross fair value	606,026	602,268
Gross assets, fair value disclosure	629,018	623,037
Liabilities
Gross derivative liability	(144,044)	(95,061)
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | US commingled equity funds
Assets
Decommissioning fund investments, gross fair value	314,957	309,620
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Cash and cash equivalents
Assets
Decommissioning fund investments, gross fair value		11,453
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | U.S. Treasury
Assets
Decommissioning fund investments, gross fair value	0
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Corporate
Assets
Decommissioning fund investments, gross fair value	96,243	109,379
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Mortgage-backed securities
Assets
Decommissioning fund investments, gross fair value	99,065	88,465
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Municipality bonds
Assets
Decommissioning fund investments, gross fair value	72,206	69,139
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2) | Other (a)
Assets
Decommissioning fund investments, gross fair value	23,555	14,212
Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3)
Assets
Gross derivative assets	30,364	32,598
Gross assets, fair value disclosure	30,364	32,598
Liabilities
Gross derivative liability	$ (63,343)	$ (73,984)