Summary of Significant Accounting Policies Cash Flow Restatements (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		9 Months Ended	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Jun. 30, 2013	Jun. 30, 2012	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Regulatory Assets	$ (28,953)	$ (70,842)	$ (66,102)	$ (110,088)	$ (130,536)	$ (170,374)	$ (64,173)	$ (69,528)
Net Cash Provided by (Used in) Operating Activities	301,265	250,947	469,253	497,625	626,140	782,685	915,923	770,223
Energy Efficiency Expenditures	0	0		0	0	0	0
Net Cash Provided by (Used in) Investing Activities	(63,820)	(203,298)	(224,058)	(426,671)	(603,145)	(692,970)	(982,410)	(810,041)
Scenario, Previously Reported [Member]
Regulatory Assets	(6,411)	(48,185)		(61,856)	(60,434)	(64,368)	30,232	26,198
Net Cash Provided by (Used in) Operating Activities	323,807	273,604		545,857	696,242	888,691	1,010,328	865,949
Energy Efficiency Expenditures	(22,542)	(22,657)		(48,232)	(70,103)	(106,006)	(94,405)	(95,726)
Net Cash Provided by (Used in) Investing Activities	(86,362)	(225,955)		(474,903)	(673,248)	(798,976)	(1,076,815)	(905,767)
Subsidiaries [Member]
Regulatory Assets		(70,842)	(66,102)	(110,088)	(130,536)	(170,374)	(65,134)	(69,528)
Net Cash Provided by (Used in) Operating Activities	309,701	238,572	491,352	518,197	647,792	797,882	809,017	480,049
Energy Efficiency Expenditures	0	0		0	0	0	0
Net Cash Provided by (Used in) Investing Activities	(63,756)	(189,109)	(223,841)	(405,896)	(582,308)	(672,069)	(966,183)	(810,041)
Subsidiaries [Member] | Scenario, Previously Reported [Member]
Regulatory Assets	(6,411)	(48,185)		(61,856)	(60,434)	(64,368)	29,271	26,198
Net Cash Provided by (Used in) Operating Activities	332,243	261,229		566,429	717,894	903,888	903,422	575,775
Energy Efficiency Expenditures	(22,542)	(22,657)		(48,232)	(70,103)	(106,006)	(94,405)	(95,726)
Net Cash Provided by (Used in) Investing Activities	$ (86,298)	$ (211,766)		$ (454,128)	$ (652,411)	$ (778,075)	$ (1,060,588)	$ (905,767)