Fair Value Measurements (Fair Value Measurements By Level For Assets Measured At Fair Value On A Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Cash and cash equivalents	$ 1,546	$ 974
U.S. Treasury securities and obligations of U.S. government corporations and agencies	377	225
Corporate securities	556	296
Municipal securities, General obligation	54	54
Municipal securities, Pre-refunded	5	11
Municipal securities, Revenue	102	69
Municipal securities, Variable rate demand notes	14	29
Asset backed securities	180	139
Mortgage backed securities	85	34
Total investments	1,373us-gaap_InvestmentsFairValueDisclosure	857us-gaap_InvestmentsFairValueDisclosure
Cash and cash equivalents	79	27
Certificates of deposit	6	6
U.S. Treasury securities and obligations of U.S. government corporations and agencies	15	14
Total restricted deposits	100	47
Interest rate swap agreements	11	10
Total assets at fair value	3,030us-gaap_AssetsFairValueDisclosure	1,888us-gaap_AssetsFairValueDisclosure
Level I [Member]
Cash and cash equivalents	1,546	974
U.S. Treasury securities and obligations of U.S. government corporations and agencies	360	213
Corporate securities	0	0
Municipal securities, General obligation	0	0
Municipal securities, Pre-refunded	0	0
Municipal securities, Revenue	0	0
Municipal securities, Variable rate demand notes	0	0
Asset backed securities	0	0
Mortgage backed securities	0	0
Total investments	360	213
Cash and cash equivalents	79	27
Certificates of deposit	6	6
U.S. Treasury securities and obligations of U.S. government corporations and agencies	15	14
Total restricted deposits	100	47
Interest rate swap agreements	0	0
Total assets at fair value	2,006	1,234
Level II [Member]
Cash and cash equivalents	0	0
U.S. Treasury securities and obligations of U.S. government corporations and agencies	17	12
Corporate securities	556	296
Municipal securities, General obligation	54	54
Municipal securities, Pre-refunded	5	11
Municipal securities, Revenue	102	69
Municipal securities, Variable rate demand notes	14	29
Asset backed securities	180	139
Mortgage backed securities	85	34
Total investments	1,013	644
Cash and cash equivalents	0	0
Certificates of deposit	0	0
U.S. Treasury securities and obligations of U.S. government corporations and agencies	0	0
Total restricted deposits	0	0
Interest rate swap agreements	11	10
Total assets at fair value	1,024	654
Level III [Member]
Cash and cash equivalents	0	0
U.S. Treasury securities and obligations of U.S. government corporations and agencies	0	0
Corporate securities	0	0
Municipal securities, General obligation	0	0
Municipal securities, Pre-refunded	0	0
Municipal securities, Revenue	0	0
Municipal securities, Variable rate demand notes	0	0
Asset backed securities	0	0
Mortgage backed securities	0	0
Total investments	0	0
Cash and cash equivalents	0	0
Certificates of deposit	0	0
U.S. Treasury securities and obligations of U.S. government corporations and agencies	0	0
Total restricted deposits	0	0
Interest rate swap agreements	0	0
Total assets at fair value	$ 0	$ 0

This element represents asset backed securities reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

No definition available.