Fair Value of Assets and Liabilities - Assets and liabilities measured at fair value on a recurring and non-recurring basis (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Assets
Fixed income securities	$ 59,044	$ 57,170
Equity securities	8,162	5,036
Short-term investments	4,256	3,027
Separate account assets	3,044	$ 2,805
Counterparty and cash collateral netting	(40)
Derivative Asset Net of Offsets and Securities Collateral	$ (40)
Assets as a percent of assets measured at fair value	(0.10%)	0.00%
Total	$ 76,461	$ 69,936
Liabilities
Liabilities as a percent of liabilities measured at fair value	(2.20%)	(2.10%)
U.S. government and agencies
Assets
Fixed income securities	$ 5,086	$ 5,517
Municipal
Assets
Fixed income securities	8,620	9,169
Foreign government
Assets
Fixed income securities	979	747
ABS
Assets
Fixed income securities	862	1,045
MBS
Assets
Fixed income securities	419	$ 534
Quoted prices in active markets for identical assets (Level 1)
Assets
Total assets at fair value	$ 17,081
Assets as a percent of assets measured at fair value	22.90%	19.90%
Liabilities
Liabilities as a percent of liabilities measured at fair value	0.00%	0.30%
Significant other observable inputs (Level 2)
Assets
Total assets at fair value	$ 56,868
Assets as a percent of assets measured at fair value	76.20%	79.10%
Liabilities
Liabilities as a percent of liabilities measured at fair value	15.70%	22.10%
Significant unobservable inputs (Level 3)
Assets
Total assets at fair value	$ 738
Assets as a percent of assets measured at fair value	1.00%	1.00%
Liabilities
Liabilities as a percent of liabilities measured at fair value	86.50%	79.70%
Investments reported at NAV
Assets
Investments measured using the net asset value practical expedient	$ 1,814	$ 1,779
Recurring
Assets
Other investments: Free-standing derivatives, Counterparty and cash collateral netting	40	23
Counterparty and cash collateral netting		(23)
Liabilities
Counterparty and cash collateral netting	12	6
Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed income securities	4,689	5,085
Equity securities	7,407	4,364
Short-term investments	1,940	1,338
Other investments: Free-standing derivatives	0	0
Separate account assets	3,044	2,805
Other assets	1	2
Total assets at fair value		13,594
Liabilities
Contractholder funds: Derivatives embedded in life and annuity contracts	0	0
Other liabilities: Free-standing derivatives	0	(1)
Total liabilities at fair value	0	(1)
Recurring | Quoted prices in active markets for identical assets (Level 1) | U.S. government and agencies
Assets
Fixed income securities	4,689	5,085
Recurring | Quoted prices in active markets for identical assets (Level 1) | Municipal
Assets
Fixed income securities	0	0
Recurring | Quoted prices in active markets for identical assets (Level 1) | Corporate - public
Assets
Fixed income securities	0	0
Recurring | Quoted prices in active markets for identical assets (Level 1) | Corporate - privately placed
Assets
Fixed income securities	0	0
Recurring | Quoted prices in active markets for identical assets (Level 1) | Foreign government
Assets
Fixed income securities	0	0
Recurring | Quoted prices in active markets for identical assets (Level 1) | ABS
Assets
Fixed income securities	0	0
Recurring | Quoted prices in active markets for identical assets (Level 1) | MBS
Assets
Fixed income securities	0	0
Recurring | Significant other observable inputs (Level 2)
Assets
Fixed income securities	54,013	51,760
Equity securities	384	331
Short-term investments	2,291	1,659
Other investments: Free-standing derivatives	180	139
Separate account assets	0	0
Other assets	0	0
Total assets at fair value		53,889
Liabilities
Contractholder funds: Derivatives embedded in life and annuity contracts	0	0
Other liabilities: Free-standing derivatives	(84)	(62)
Total liabilities at fair value	(84)	(62)
Recurring | Significant other observable inputs (Level 2) | U.S. government and agencies
Assets
Fixed income securities	397	432
Recurring | Significant other observable inputs (Level 2) | Municipal
Assets
Fixed income securities	8,558	9,099
Recurring | Significant other observable inputs (Level 2) | Corporate - public
Assets
Fixed income securities	30,819	29,200
Recurring | Significant other observable inputs (Level 2) | Corporate - privately placed
Assets
Fixed income securities	12,084	10,798
Recurring | Significant other observable inputs (Level 2) | Foreign government
Assets
Fixed income securities	979	747
Recurring | Significant other observable inputs (Level 2) | ABS
Assets
Fixed income securities	797	976
Recurring | Significant other observable inputs (Level 2) | MBS
Assets
Fixed income securities	379	508
Recurring | Significant unobservable inputs (Level 3)
Assets
Fixed income securities	342	325
Equity securities	371	341
Short-term investments	25	30
Other investments: Free-standing derivatives	0	1
Separate account assets	0	0
Other assets	0	0
Total assets at fair value		697
Liabilities
Contractholder funds: Derivatives embedded in life and annuity contracts	(462)	(224)
Other liabilities: Free-standing derivatives	0	0
Total liabilities at fair value	(462)	(224)
Recurring | Significant unobservable inputs (Level 3) | U.S. government and agencies
Assets
Fixed income securities	0	0
Recurring | Significant unobservable inputs (Level 3) | Municipal
Assets
Fixed income securities	62	70
Recurring | Significant unobservable inputs (Level 3) | Corporate - public
Assets
Fixed income securities	61	70
Recurring | Significant unobservable inputs (Level 3) | Corporate - privately placed
Assets
Fixed income securities	114	90
Recurring | Significant unobservable inputs (Level 3) | Foreign government
Assets
Fixed income securities	0	0
Recurring | Significant unobservable inputs (Level 3) | ABS
Assets
Fixed income securities	65	69
Recurring | Significant unobservable inputs (Level 3) | MBS
Assets
Fixed income securities	40	$ 26
Fair value
Assets
Total assets at fair value	$ 74,647
Assets as a percent of assets measured at fair value	100.00%	100.00%
Liabilities
Liabilities as a percent of liabilities measured at fair value	100.00%	100.00%
Fair value | Recurring
Assets
Fixed income securities	$ 59,044	$ 57,170
Equity securities	8,162	5,036
Short-term investments	4,256	3,027
Other investments: Free-standing derivatives	140	117
Separate account assets	3,044	2,805
Other assets	1	2
Total assets at fair value		68,157
Liabilities
Contractholder funds: Derivatives embedded in life and annuity contracts	(462)	(224)
Other liabilities: Free-standing derivatives	(72)	(57)
Total liabilities at fair value	(534)	(281)
Fair value | Recurring | U.S. government and agencies
Assets
Fixed income securities	5,086	5,517
Fair value | Recurring | Municipal
Assets
Fixed income securities	8,620	9,169
Fair value | Recurring | Corporate - public
Assets
Fixed income securities	30,880	29,270
Fair value | Recurring | Corporate - privately placed
Assets
Fixed income securities	12,198	10,888
Fair value | Recurring | Foreign government
Assets
Fixed income securities	979	747
Fair value | Recurring | ABS
Assets
Fixed income securities	862	1,045
Fair value | Recurring | MBS
Assets
Fixed income securities	$ 419	$ 534