Fair Value Measurements - Changes in balances of Level 3 financial assets carried at fair value (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	$ 3,521	$ 2,290
Total realized/unrealized gains (losses) included in Net income	4	(13)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	39	(12)
Purchases and issuances	508	466
Sales and settlements	(211)	(79)
Transfer into Level 3	16	36
Transfer out of Level 3	(192)	(88)
Financial assets, Ending Balance	3,685	2,600
Fixed maturities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	3,164	2,102
Total realized/unrealized gains (losses) included in Net income	4	(6)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	39	(12)
Purchases and issuances	507	454
Sales and settlements	(211)	(75)
Transfer into Level 3	0	7
Transfer out of Level 3	(192)	(88)
Financial assets, Ending Balance	3,311	2,382
U.S. government agency
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	9	8
Total realized/unrealized gains (losses) included in Net income	0	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0
Purchases and issuances	0	0
Sales and settlements	(1)	0
Transfer into Level 3	0	0
Transfer out of Level 3	0	0
Financial assets, Ending Balance	8	8
State and municipal
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	59	148
Total realized/unrealized gains (losses) included in Net income	0	0
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	5	(1)
Purchases and issuances	0	0
Sales and settlements	(1)	(1)
Transfer into Level 3	0	0
Transfer out of Level 3	0	(84)
Financial assets, Ending Balance	63	62
Residential MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	197	122
Total realized/unrealized gains (losses) included in Net income	5	(4)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	(5)	0
Purchases and issuances	0	0
Sales and settlements	(6)	(6)
Transfer into Level 3	0	7
Transfer out of Level 3	(22)	(4)
Financial assets, Ending Balance	169	115
Commercial MBS
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	56	36
Total realized/unrealized gains (losses) included in Net income	0	(1)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0
Purchases and issuances	0	12
Sales and settlements	(1)	0
Transfer into Level 3	0	0
Transfer out of Level 3	0	0
Financial assets, Ending Balance	55	47
Asset-backed securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	847	744
Total realized/unrealized gains (losses) included in Net income	(3)	(2)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	8	3
Purchases and issuances	75	204
Sales and settlements	(114)	(37)
Transfer into Level 3	0	0
Transfer out of Level 3	(143)	0
Financial assets, Ending Balance	670	912
Corporate and other
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	1,996	1,044
Total realized/unrealized gains (losses) included in Net income	2	1
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	31	(14)
Purchases and issuances	432	238
Sales and settlements	(88)	(31)
Transfer into Level 3	0	0
Transfer out of Level 3	(27)	0
Financial assets, Ending Balance	2,346	1,238
Equity securities
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	336	165
Total realized/unrealized gains (losses) included in Net income	1	(5)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0
Purchases and issuances	1	9
Sales and settlements	0	(4)
Transfer into Level 3	16	29
Transfer out of Level 3	0	0
Financial assets, Ending Balance	354	194
Assets of MIE
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Financial assets, Beginning Balance	21	23
Total realized/unrealized gains (losses) included in Net income	(1)	(2)
Total realized/unrealized gains (losses) included in other comprehensive income (loss)	0	0
Purchases and issuances	0	3
Sales and settlements	0	0
Transfer into Level 3	0	0
Transfer out of Level 3	0	0
Financial assets, Ending Balance	$ 20	$ 24

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of gain (loss) recognized in other comprehensive income (loss) for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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- Definition

Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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