Investments - Securities with a Decline in Fair Value (Details) - Debt Securities [Member] $ in Millions	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Investment Grade [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fair value, one year or less	$ 506	$ 7,127
Amortized cost, one year or less	513	7,367
Unrealized depreciation, one year or less	(7)	(240)
Fair value, more than one year	610	3,023
Amortized cost, more than one year	632	3,181
Unrealized depreciation, more than one year	$ (22)	$ (158)
Investment Grade [Member] \| One Year Or Less [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Number of issues, total	165	1,324
Investment Grade [Member] \| More Than One Year [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Number of issues, total	185	784
Below Investment Grade [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Fair value, one year or less	$ 254	$ 1,185
Amortized cost, one year or less	258	1,240
Unrealized depreciation, one year or less	(4)	(55)
Fair value, more than one year	174	249
Amortized cost, more than one year	180	270
Unrealized depreciation, more than one year	$ (6)	$ (21)
Below Investment Grade [Member] \| One Year Or Less [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Number of issues, total	470	1,190
Below Investment Grade [Member] \| More Than One Year [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Number of issues, total	111	245

No definition available.

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- Definition

This item represents the aggregate amortized cost of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

No definition available.

+ Details

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- Definition

+ References

No definition available.

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- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

+ References

No definition available.

+ Details

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- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

+ References

No definition available.

+ Details

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- Definition

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

No definition available.

+ Details

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- Definition

+ References

No definition available.

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- Definition

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27290-111563

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