Fair Value of Assets and Liabilities - Assets and liabilities measured at fair value (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets
Fixed income securities	$ 21,725	$ 21,400
Equity securities	1,469	1,325
Short-term investments	1,191	810
Other investments:
Other investments: Free-standing derivatives		1	$ 1
Counterparty and cash collateral netting	(10)	(8)
Separate account assets	3,009	2,783
Derivative asset, fair value	$ (10)	$ (8)
% of total assets at fair value	0.00%	0.00%
Total	$ 28,291	$ 27,128
Liabilities
Counterparty and cash collateral netting	3	2
Total liabilities at fair value, Counterparty and cash collateral netting	$ 3	$ 2
% of total liabilities at fair value	(0.60%)	(0.90%)
Commitments to invest in limited partnership interests	$ 1,063	$ 1,195
Fair value
Assets
Fixed income securities	21,725	21,400
Equity securities	1,469	1,325
Short-term investments	1,191	810
Other investments:
Other investments: Free-standing derivatives	124	23
Separate account assets	3,009	2,783
Total assets at fair value	$ 27,518	$ 26,341
% of total assets at fair value	100.00%	100.00%
Liabilities
Contractholder funds: Derivatives embedded in life and annuity contracts	$ (427)	$ (223)
Other liabilities: Free-standing derivatives	(62)	(5)
Total recurring basis liabilities at fair value	$ (489)	$ (228)
% of total liabilities at fair value	100.00%	100.00%
Limited partnership interests
Liabilities
Commitments to invest in limited partnership interests	$ 201
U.S. government and agencies
Assets
Fixed income securities	882	$ 773
U.S. government and agencies | Fair value
Assets
Fixed income securities	882	773
Municipal
Assets
Fixed income securities	1,755	2,195
Municipal | Fair value
Assets
Fixed income securities	1,755	2,195
Corporate - public | Fair value
Assets
Fixed income securities	12,802	11,924
Corporate - privately placed | Fair value
Assets
Fixed income securities	5,639	5,663
Foreign government
Assets
Fixed income securities	149	179
Foreign government | Fair value
Assets
Fixed income securities	149	179
Asset-backed securities
Assets
Fixed income securities	317	429
Asset-backed securities | Fair value
Assets
Fixed income securities	317	429
MBS
Assets
Fixed income securities	181	237
MBS | Fair value
Assets
Fixed income securities	181	237
Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed income securities	590	493
Equity securities	1,340	1,182
Short-term investments	493	443
Other investments:
Other investments: Free-standing derivatives	0	0
Separate account assets	3,009	2,783
Total assets at fair value	$ 5,432	$ 4,901
% of total assets at fair value	19.70%	18.60%
Liabilities
Contractholder funds: Derivatives embedded in life and annuity contracts	$ 0	$ 0
Other liabilities: Free-standing derivatives	0	0
Total recurring basis liabilities at fair value	$ 0	$ 0
% of total liabilities at fair value	0.00%	0.00%
Quoted prices in active markets for identical assets (Level 1) | U.S. government and agencies
Assets
Fixed income securities	$ 590	$ 493
Quoted prices in active markets for identical assets (Level 1) | Municipal
Assets
Fixed income securities	0	0
Quoted prices in active markets for identical assets (Level 1) | Corporate - public
Assets
Fixed income securities	0	0
Quoted prices in active markets for identical assets (Level 1) | Corporate - privately placed
Assets
Fixed income securities	0	0
Quoted prices in active markets for identical assets (Level 1) | Foreign government
Assets
Fixed income securities	0	0
Quoted prices in active markets for identical assets (Level 1) | Asset-backed securities
Assets
Fixed income securities	0	0
Quoted prices in active markets for identical assets (Level 1) | MBS
Assets
Fixed income securities	0	0
Significant other observable inputs (Level 2)
Assets
Fixed income securities	20,927	20,716
Equity securities	13	14
Short-term investments	698	367
Other investments:
Other investments: Free-standing derivatives	134	30
Separate account assets	0	0
Total assets at fair value	$ 21,772	$ 21,127
% of total assets at fair value	79.10%	80.20%
Liabilities
Contractholder funds: Derivatives embedded in life and annuity contracts	$ 0	$ 0
Other liabilities: Free-standing derivatives	(65)	(7)
Total recurring basis liabilities at fair value	$ (65)	$ (7)
% of total liabilities at fair value	13.30%	3.10%
Significant other observable inputs (Level 2) | U.S. government and agencies
Assets
Fixed income securities	$ 292	$ 280
Significant other observable inputs (Level 2) | Municipal
Assets
Fixed income securities	1,715	2,156
Significant other observable inputs (Level 2) | Corporate - public
Assets
Fixed income securities	12,777	11,891
Significant other observable inputs (Level 2) | Corporate - privately placed
Assets
Fixed income securities	5,517	5,566
Significant other observable inputs (Level 2) | Foreign government
Assets
Fixed income securities	149	179
Significant other observable inputs (Level 2) | Asset-backed securities
Assets
Fixed income securities	301	407
Significant other observable inputs (Level 2) | MBS
Assets
Fixed income securities	176	237
Significant unobservable inputs (Level 3)
Assets
Fixed income securities	208	191
Equity securities	116	129
Short-term investments	0	0
Other investments:
Other investments: Free-standing derivatives	0	1
Separate account assets	0	0
Total assets at fair value	$ 324	$ 321
% of total assets at fair value	1.20%	1.20%
Liabilities
Contractholder funds: Derivatives embedded in life and annuity contracts	$ (427)	$ (223)
Other liabilities: Free-standing derivatives	0	0
Total recurring basis liabilities at fair value	$ (427)	$ (223)
% of total liabilities at fair value	87.30%	97.80%
Significant unobservable inputs (Level 3) | U.S. government and agencies
Assets
Fixed income securities	$ 0	$ 0
Significant unobservable inputs (Level 3) | Municipal
Assets
Fixed income securities	40	39
Significant unobservable inputs (Level 3) | Corporate - public
Assets
Fixed income securities	25	33
Significant unobservable inputs (Level 3) | Corporate - privately placed
Assets
Fixed income securities	122	97
Significant unobservable inputs (Level 3) | Foreign government
Assets
Fixed income securities	0	0
Significant unobservable inputs (Level 3) | Asset-backed securities
Assets
Fixed income securities	16	22
Significant unobservable inputs (Level 3) | MBS
Assets
Fixed income securities	5	0
Fair Value Measured at Net Asset Value Per Share [Member]
Other investments:
Investments reported at NAV	$ 773	$ 787
Minimum | Limited partnership interests
Fair value of assets and liabilities measured on recurring and non-recurring basis
Useful life	10 years
Distribution and proceed period	4 years
Maximum | Limited partnership interests
Fair value of assets and liabilities measured on recurring and non-recurring basis
Useful life	12 years
Distribution and proceed period	9 years