Investments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Schedule Of Available For Sale Securities [Abstract]
Fair Value	$ 1,904		$ 1,904		$ 1,650
Unrealized Losses	23		23		31
Unrealized Gains	573		573		444
Fair Value Of Available For Sale Debt Securities By Contractual Maturity [Abstract]
Due in one year or less	13		13
Due after one through five years	125		125
Due after five through 10 years	136		136
Due after 10 years	301		301
Total	575		575
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	121		121
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	14		14
Available-for-sale securities, continuous unrealized loss position, less than twelve months, aggregate losses	9		9
Sales Of Available For Sale Securities [Abstract]
Proceeds	274	1,062	794	4,254
Realized gains	8	9	23	24
Realized losses	6	11	12	20
PEC [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	1,229		1,229		1,107
Unrealized Losses	15		15		20
Unrealized Gains	368		368		287
Fair Value Of Available For Sale Debt Securities By Contractual Maturity [Abstract]
Due in one year or less	11		11
Due after one through five years	102		102
Due after five through 10 years	72		72
Due after 10 years	194		194
Total	379		379
Sales Of Available For Sale Securities [Abstract]
Proceeds	101	136	351	386
Realized gains	4	4	14	10
Realized losses	2	4	5	9
PEF [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	662		662		530
Unrealized Losses	8		8		11
Unrealized Gains	205		205		157
Fair Value Of Available For Sale Debt Securities By Contractual Maturity [Abstract]
Due in one year or less	2		2
Due after one through five years	18		18
Due after five through 10 years	55		55
Due after 10 years	82		82
Total	157		157
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	47		47
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	4		4
Available-for-sale securities, continuous unrealized loss position, less than twelve months, aggregate losses	4		4
Sales Of Available For Sale Securities [Abstract]
Proceeds	173	926	443	3,861
Realized gains	4	5	9	14
Realized losses	4	7	7	11
Ndtf [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	1,844		1,844		1,630
Unrealized Losses	23		23		31
Unrealized Gains	573		573		444
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	117		117		136
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	14		14		12
Available-for-sale securities, continuous unrealized loss position, less than twelve months, aggregate losses	9		9		19
Ndtf [Member] | PEC [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	1,223		1,223		1,101
Unrealized Losses	15		15		20
Unrealized Gains	368		368		287
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	74		74		98
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	10		10		10
Available-for-sale securities, continuous unrealized loss position, less than twelve months, aggregate losses	5		5		10
Ndtf [Member] | PEF [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	621		621		529
Unrealized Losses	8		8		11
Unrealized Gains	205		205		157
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	43		43		38
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	4		4		2
Available-for-sale securities, continuous unrealized loss position, less than twelve months, aggregate losses	4		4		9
Equity Securities [Member] | Ndtf [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	1,226		1,226		1,062
Unrealized Losses	21		21		29
Unrealized Gains	534		534		412
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	89		89		112
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	13		13		10
Available-for-sale securities, continuous unrealized loss position, less than twelve months, aggregate losses	8		8		19
Equity Securities [Member] | Ndtf [Member] | PEC [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	797		797		690
Unrealized Losses	15		15		20
Unrealized Gains	340		340		262
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	56		56		69
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	10		10		10
Available-for-sale securities, continuous unrealized loss position, less than twelve months, aggregate losses	5		5		10
Equity Securities [Member] | Ndtf [Member] | PEF [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	429		429		372
Unrealized Losses	6		6		9
Unrealized Gains	194		194		150
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	33		33		43
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	3		3
Available-for-sale securities, continuous unrealized loss position, less than twelve months, aggregate losses	3		3		9
Corporate Debt Securities [Member] | Ndtf [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	85		85		86
Unrealized Gains	9		9		6
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	3		3		20
Corporate Debt Securities [Member] | Ndtf [Member] | PEC [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	75		75		69
Unrealized Gains	8		8		5
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	2		2		10
Corporate Debt Securities [Member] | Ndtf [Member] | PEF [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	10		10		17
Unrealized Gains	1		1		1
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value					10
Municipal Bonds [Member] | Ndtf [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	134		134		127
Unrealized Losses	1		1		2
Unrealized Gains	10		10		7
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	14		14		21
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	1		1		2
Municipal Bonds [Member] | Ndtf [Member] | PEC [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	61		61		55
Unrealized Gains	4		4		3
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	5		5		8
Municipal Bonds [Member] | Ndtf [Member] | PEF [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	73		73		72
Unrealized Losses	1		1		2
Unrealized Gains	6		6		4
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	9		9		13
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, aggregate losses	1		1		2
Municipal Bonds [Member] | Other Classification [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	39		39
Municipal Bonds [Member] | Other Classification [Member] | PEF [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	39		39
U.S. Government Bonds [Member] | Ndtf [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	295		295		268
Unrealized Gains	17		17		18
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	10		10		(23)
U.S. Government Bonds [Member] | Ndtf [Member] | PEC [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	230		230		225
Unrealized Gains	15		15		16
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	11		11		9
U.S. Government Bonds [Member] | Ndtf [Member] | PEF [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	65		65		43
Unrealized Gains	2		2		2
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value					(32)
Other [Member] | Ndtf [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	104		104		87
Unrealized Losses	1		1
Unrealized Gains	3		3		1
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	1		1		6
Available-for-sale securities, continuous unrealized loss position, less than twelve months, aggregate losses	1		1
Other [Member] | Ndtf [Member] | PEC [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	60		60		62
Unrealized Gains	1		1		1
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value					2
Other [Member] | Ndtf [Member] | PEF [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	44		44		25
Unrealized Losses	1		1
Unrealized Gains	2		2
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	1		1		4
Available-for-sale securities, continuous unrealized loss position, less than twelve months, aggregate losses	1		1
Other [Member] | Other Classification [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	21		21		20
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	4		4
Other [Member] | Other Classification [Member] | PEC [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	6		6		6
Other [Member] | Other Classification [Member] | PEF [Member]
Schedule Of Available For Sale Securities [Abstract]
Fair Value	2		2		1
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale securities, continuous unrealized loss position, fair value	$ 4		$ 4