Fair Value of Financial Assets and Liabilities, Changes in Level 3 Nuclear Decommissioning Fund (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period	$ 165,486		$ 120,064
Purchases	16,243		12,429
Settlements	(1,381)		0
Gains recognized as regulatory assets	1,045	[1]	4,262	[1]
Balance at end of period	181,393		136,755
Private equity investments
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period	101,237		62,696
Purchases	12,382		8,769
Settlements	0		0
Gains recognized as regulatory assets	0	[1]	2,336	[1]
Balance at end of period	113,619		73,801
Real estate
Changes in Level 3 Nuclear Decommissioning Fund Assets [Roll Forward]
Balance at beginning of period	64,249		57,368
Purchases	3,861		3,660
Settlements	(1,381)		0
Gains recognized as regulatory assets	1,045	[1]	1,926	[1]
Balance at end of period	$ 67,774		$ 62,954

[1] Gains are deferred as a component of the regulatory assets for nuclear decommissioning.