Investments (Short-Term and Long-Term Investments) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 32,448	$ 32,240
Securities, Available for sale Debt Securities, Gross Unrealized Gains	325	101
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(143)	(443)
Debt Securities, Available-for-sale	32,630	31,898
Held-to-maturity securities, Amortized Cost	724	621
Securities, Held to maturity, Unrecognized Holding Gain	2	1
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(1)	(2)
Debt Securities, Held-to-maturity, Fair Value	725	620
Total debt securities, Amortized Cost	33,172	32,861
Total debt securities, Gross Unrealized Gains	327	102
Total debt securities, Gross Unrealized Losses	(144)	(445)
Debt securities, Fair Value Disclosure	33,355	32,518
Debt Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale	32,630	31,898
U.S. Government and Agency Obligations [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	3,610	3,434
Securities, Available for sale Debt Securities, Gross Unrealized Gains	30	13
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(19)	(42)
Debt Securities, Available-for-sale	3,621	3,405
Held-to-maturity securities, Amortized Cost	265	255
Securities, Held to maturity, Unrecognized Holding Gain	0	1
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(1)	(2)
Debt Securities, Held-to-maturity, Fair Value	264	254
State and Municipal Obligations [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	6,566	7,117
Securities, Available for sale Debt Securities, Gross Unrealized Gains	150	61
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(9)	(57)
Debt Securities, Available-for-sale	6,707	7,121
Held-to-maturity securities, Amortized Cost	31	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	32	11
Corporate Obligations [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	15,589	15,366
Securities, Available for sale Debt Securities, Gross Unrealized Gains	95	14
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(58)	(218)
Debt Securities, Available-for-sale	15,626	15,162
Held-to-maturity securities, Amortized Cost	428	355
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	429	355
U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	5,212	4,947
Securities, Available for sale Debt Securities, Gross Unrealized Gains	37	11
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(51)	(106)
Debt Securities, Available-for-sale	5,198	4,852
Non-U.S. Agency Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	1,471	1,376
Securities, Available for sale Debt Securities, Gross Unrealized Gains	13	2
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(6)	(20)
Debt Securities, Available-for-sale	$ 1,478	$ 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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Amortized cost of 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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