Investments (Short-Term and Long-Term Investments) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	$ 28,410	$ 24,330
Securities, Available for sale Debt Securities, Gross Unrealized Gains	167	92
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(153)	(243)
Available-for-sale Securities, Debt Securities	28,424	24,179
Available-for-sale Equity Securities, Amortized Cost Basis	2,026	2,036
Securities, Available for sale Equity Securities, Gross Unrealized Gains	7	52
Securities, Available for sale Equity Securities, Gross Unrealized Losses	(41)	(47)
Available-for-sale Securities, Equity Securities	1,992	2,041
Held-to-maturity securities, Amortized Cost	536	493
Securities, Held to maturity, Unrecognized Holding Gain	1	1
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	(1)	0
Held-to-maturity Securities, Fair Value	536	494
Total investments, Amortized Cost	30,972	26,859
Total investments, Gross Unrealized Gains	175	145
Total investments, Gross Unrealized Losses	(195)	(290)
Investments, Fair Value Disclosure	30,952	26,714
Debt Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Debt Securities	28,424	24,179
U.S. Government and Agency Obligations [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	2,673	2,294
Securities, Available for sale Debt Securities, Gross Unrealized Gains	1	1
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(30)	(31)
Available-for-sale Securities, Debt Securities	2,644	2,264
Held-to-maturity securities, Amortized Cost	254	250
Securities, Held to maturity, Unrecognized Holding Gain	1	1
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	(1)	0
Held-to-maturity Securities, Fair Value	254	251
State and Municipal Obligations [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	7,596	7,120
Securities, Available for sale Debt Securities, Gross Unrealized Gains	99	40
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(35)	(101)
Available-for-sale Securities, Debt Securities	7,660	7,059
Held-to-maturity securities, Amortized Cost	2	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	2	5
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	13,181	10,944
Securities, Available for sale Debt Securities, Gross Unrealized Gains	57	41
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(44)	(58)
Available-for-sale Securities, Debt Securities	13,194	10,927
Held-to-maturity securities, Amortized Cost	280	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	280	238
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	3,942	2,963
Securities, Available for sale Debt Securities, Gross Unrealized Gains	7	7
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(38)	(43)
Available-for-sale Securities, Debt Securities	3,911	2,927
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Debt Securities, Amortized Cost Basis	1,018	1,009
Securities, Available for sale Debt Securities, Gross Unrealized Gains	3	3
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(6)	(10)
Available-for-sale Securities, Debt Securities	$ 1,015	$ 1,002