Investments (Short-Term and Long-Term Investments) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 23,302	$ 18,539
Securities, Available for sale Debt Securities, Gross Unrealized Gains	467	209
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(15)	(117)
Available-for-sale Securities, Debt Securities	23,754	18,631
Available-for-sale Equity Securities, Amortized Cost Basis	1,919	1,638
Securities, Available for sale Equity Securities, Gross Unrealized Gains	58	58
Securities, Available for sale Equity Securities, Gross Unrealized Losses	(41)	(57)
Available-for-sale Securities, Equity Securities	1,936	1,639
Held-to-maturity securities, Amortized Cost	505	510
Securities, Held to maturity, Unrecognized Holding Gain	2	1
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held-to-maturity Securities, Fair Value	507	511
Total investments, Amortized Cost	25,726	20,687
Total investments, Gross Unrealized Gains	527	268
Total investments, Gross Unrealized Losses	(56)	(174)
Investments, Fair Value Disclosure	26,197	20,781
Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	23,754	18,631
U.S. Government and Agency Obligations [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,301	1,982
Securities, Available for sale Debt Securities, Gross Unrealized Gains	12	1
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(2)	(6)
Available-for-sale Securities, Debt Securities	2,311	1,977
Held-to-maturity securities, Amortized Cost	174	163
Securities, Held to maturity, Unrecognized Holding Gain	2	1
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held-to-maturity Securities, Fair Value	176	164
State and Municipal Obligations [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	7,207	6,022
Securities, Available for sale Debt Securities, Gross Unrealized Gains	213	149
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(3)	(3)
Available-for-sale Securities, Debt Securities	7,417	6,168
Held-to-maturity securities, Amortized Cost	8	8
Securities, Held to maturity, Unrecognized Holding Gain	0	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held-to-maturity Securities, Fair Value	8	8
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	10,292	7,446
Securities, Available for sale Debt Securities, Gross Unrealized Gains	176	41
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(6)	(81)
Available-for-sale Securities, Debt Securities	10,462	7,406
Held-to-maturity securities, Amortized Cost	323	339
Securities, Held to maturity, Unrecognized Holding Gain	0	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held-to-maturity Securities, Fair Value	323	339
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,571	2,127
Securities, Available for sale Debt Securities, Gross Unrealized Gains	42	13
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(1)	(16)
Available-for-sale Securities, Debt Securities	2,612	2,124
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	931	962
Securities, Available for sale Debt Securities, Gross Unrealized Gains	24	5
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(3)	(11)
Available-for-sale Securities, Debt Securities	$ 952	$ 956

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