Investments (Short-Term and Long-Term Investments) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 26,354	$ 24,330
Securities, Available for sale Debt Securities, Gross Unrealized Gains	199	92
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(119)	(243)
Available-for-sale Securities, Debt Securities	26,434	24,179
Equity Securities, Amortized Cost Basis	2,165	2,036
Equity Securities, Gross Unrealized Gains	42	52
Equity Securities, Gross Unrealized Losses	(40)	(47)
Equity Securities, Fair Value	2,167	2,041
Held-to-maturity securities, Amortized Cost	556	493
Securities, Held to maturity, Unrecognized Holding Gain	1	1
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held-to-maturity Securities, Fair Value	557	494
Total investments, Amortized Cost	29,075	26,859
Total investments, Gross Unrealized Gains	242	145
Total investments, Gross Unrealized Losses	(159)	(290)
Investments, Fair Value Disclosure	29,158	26,714
Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	26,434	24,179
U.S. Government and Agency Obligations [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,557	2,294
Securities, Available for sale Debt Securities, Gross Unrealized Gains	3	1
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(23)	(31)
Available-for-sale Securities, Debt Securities	2,537	2,264
Held-to-maturity securities, Amortized Cost	256	250
Securities, Held to maturity, Unrecognized Holding Gain	1	1
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held-to-maturity Securities, Fair Value	257	251
State and Municipal Obligations [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	6,771	7,120
Securities, Available for sale Debt Securities, Gross Unrealized Gains	106	40
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(27)	(101)
Available-for-sale Securities, Debt Securities	6,850	7,059
Held-to-maturity securities, Amortized Cost	5	5
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	5	5
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	12,402	10,944
Securities, Available for sale Debt Securities, Gross Unrealized Gains	75	41
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(30)	(58)
Available-for-sale Securities, Debt Securities	12,447	10,927
Held-to-maturity securities, Amortized Cost	295	238
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	295	238
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	3,629	2,963
Securities, Available for sale Debt Securities, Gross Unrealized Gains	10	7
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(34)	(43)
Available-for-sale Securities, Debt Securities	3,605	2,927
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	995	1,009
Securities, Available for sale Debt Securities, Gross Unrealized Gains	5	3
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(5)	(10)
Available-for-sale Securities, Debt Securities	$ 995	$ 1,002

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