Investments (Short-Term and Long-Term Investments) (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 22,139	$ 18,539
Securities, Available for sale Debt Securities, Gross Unrealized Gains	525	209
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(16)	(117)
Available-for-sale Securities, Debt Securities	22,648	18,631
Available-for-sale Equity Securities, Amortized Cost Basis	1,804	1,638
Securities, Available for sale Equity Securities, Gross Unrealized Gains	52	58
Securities, Available for sale Equity Securities, Gross Unrealized Losses	(45)	(57)
Available-for-sale Securities, Equity Securities	1,811	1,639
Held-to-maturity securities, Amortized Cost	509	510
Securities, Held to maturity, Unrecognized Holding Gain	3	1
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held-to-maturity Securities, Fair Value	512	511
Total investments, Amortized Cost	24,452	20,687
Total investments, Gross Unrealized Gains	580	268
Total investments, Gross Unrealized Losses	(61)	(174)
Investments, Fair Value Disclosure	24,971	20,781
Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	22,648	18,631
U.S. Government and Agency Obligations [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,401	1,982
Securities, Available for sale Debt Securities, Gross Unrealized Gains	24	1
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	0	(6)
Available-for-sale Securities, Debt Securities	2,425	1,977
Held-to-maturity securities, Amortized Cost	175	163
Securities, Held to maturity, Unrecognized Holding Gain	3	1
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held-to-maturity Securities, Fair Value	178	164
State and Municipal Obligations [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	6,749	6,022
Securities, Available for sale Debt Securities, Gross Unrealized Gains	260	149
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(1)	(3)
Available-for-sale Securities, Debt Securities	7,008	6,168
Held-to-maturity securities, Amortized Cost	7	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	7	8
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	9,479	7,446
Securities, Available for sale Debt Securities, Gross Unrealized Gains	170	41
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(11)	(81)
Available-for-sale Securities, Debt Securities	9,638	7,406
Held-to-maturity securities, Amortized Cost	327	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	327	339
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,580	2,127
Securities, Available for sale Debt Securities, Gross Unrealized Gains	44	13
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(1)	(16)
Available-for-sale Securities, Debt Securities	2,623	2,124
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	930	962
Securities, Available for sale Debt Securities, Gross Unrealized Gains	27	5
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(3)	(11)
Available-for-sale Securities, Debt Securities	$ 954	$ 956

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