Fair Value of Financial Assets and Liabilities, Changes in Level 3 Nuclear Decommissioning Fund (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period	$ 136,755	$ 74,912	$ 120,064	$ 104,600
Purchases	2,381	9,330	14,809	15,372
Settlements	0	(4,723)	0	(9,022)
Gains recognized as regulatory assets	7,645	4,211	11,908	5,056
Transfers out of Level 3	0	0	0	(32,276)	[1]
Balance at end of period	146,781	83,730	146,781	83,730
Private equity investments
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period	73,801	34,506	62,696	33,250
Purchases	2,184	7,298	10,953	8,554
Settlements	0	0	0	0
Gains recognized as regulatory assets	5,138	3,786	7,474	3,786
Transfers out of Level 3	0	0	0	0
Balance at end of period	81,123	45,590	81,123	45,590
Real estate
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period	62,954	40,406	57,368	39,074
Purchases	197	2,032	3,856	6,818
Settlements	0	(4,723)	0	(9,022)
Gains recognized as regulatory assets	2,507	425	4,434	1,270
Transfers out of Level 3	0	0	0	0
Balance at end of period	65,658	38,140	65,658	38,140
Asset-backed securities
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period				2,067
Purchases				0
Settlements				0
Gains recognized as regulatory assets				0
Transfers out of Level 3				(2,067)	[1]
Balance at end of period		0		0
Mortgage-backed securities
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period				30,209
Purchases				0
Settlements				0
Gains recognized as regulatory assets				0
Transfers out of Level 3				(30,209)	[1]
Balance at end of period		$ 0		$ 0

[1] Transfers out of Level 3 into Level 2 were principally due to diminished use of unobservable inputs that were previously significant to these fair value measurements.