Investments (Short-Term and Long-Term Investments) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 35,118	$ 32,240
Securities, Available for sale Debt Securities, Gross Unrealized Gains	852	101
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(21)	(443)
Debt Securities, Available-for-sale	35,949	31,898
Held-to-maturity securities, Amortized Cost	851	621
Securities, Held to maturity, Unrecognized Holding Gain	3	1
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	(2)
Debt Securities, Held-to-maturity, Fair Value	854	620
Total debt securities, Amortized Cost	35,969	32,861
Total debt securities, Gross Unrealized Gains	855	102
Total debt securities, Gross Unrealized Losses	(21)	(445)
Debt securities, Fair Value Disclosure	36,803	32,518
Debt Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale	35,949	31,898
U.S. Government and Agency Obligations [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	3,591	3,434
Securities, Available for sale Debt Securities, Gross Unrealized Gains	83	13
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(2)	(42)
Debt Securities, Available-for-sale	3,672	3,405
Held-to-maturity securities, Amortized Cost	272	255
Securities, Held to maturity, Unrecognized Holding Gain	2	1
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	(2)
Debt Securities, Held-to-maturity, Fair Value	274	254
State and Municipal Obligations [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	5,657	7,117
Securities, Available for sale Debt Securities, Gross Unrealized Gains	256	61
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(3)	(57)
Debt Securities, Available-for-sale	5,910	7,121
Held-to-maturity securities, Amortized Cost	32	11
Securities, Held to maturity, Unrecognized Holding Gain	1	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt Securities, Held-to-maturity, Fair Value	33	11
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	17,824	15,366
Securities, Available for sale Debt Securities, Gross Unrealized Gains	352	14
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(9)	(218)
Debt Securities, Available-for-sale	18,167	15,162
Held-to-maturity securities, Amortized Cost	547	355
Securities, Held to maturity, Unrecognized Holding Gain	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt Securities, Held-to-maturity, Fair Value	547	355
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	6,361	4,947
Securities, Available for sale Debt Securities, Gross Unrealized Gains	113	11
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(6)	(106)
Debt Securities, Available-for-sale	6,468	4,852
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	1,685	1,376
Securities, Available for sale Debt Securities, Gross Unrealized Gains	48	2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(1)	(20)
Debt Securities, Available-for-sale	$ 1,732	$ 1,358

Investments at amortized cost.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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