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, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Fixed income securities	$ 58,992	$ 57,839
Equity securities	6,621	5,666
Short-term investments	1,944	4,288
Other investments: Free-standing derivatives	1	1	$ 1
Separate account assets	3,444	3,393
Counterparty and cash collateral netting	$ (6)	$ (9)
Assets as a percent of assets measured at fair value	0.00%	0.00%
Liabilities
Other liabilities: Free-standing derivatives		$ (3)	$ (8)
Counterparty and cash collateral netting	$ 14	$ 28
Liabilities as a percent of liabilities measured at fair value	(3.90%)	(8.40%)
Limited partnership interests	$ 3	$ 24
U.S. government and agencies
Assets
Fixed income securities	3,616	3,637
Municipal
Assets
Fixed income securities	8,328	7,333
Foreign government
Assets
Fixed income securities	1,021	1,075
RMBS
Assets
Fixed income securities	578	728
CMBS
Assets
Fixed income securities	128	270
Redeemable preferred stock
Assets
Fixed income securities	23	24
Quoted prices in active markets for identical assets (Level 1)
Assets
Total assets at fair value	$ 12,819	$ 12,408
Assets as a percent of assets measured at fair value	18.00%	17.40%
Liabilities
Liabilities as a percent of liabilities measured at fair value	0.30%	0.30%
Significant other observable inputs (Level 2)
Assets
Total assets at fair value	$ 57,478	$ 58,161
Assets as a percent of assets measured at fair value	80.80%	81.50%
Liabilities
Liabilities as a percent of liabilities measured at fair value	23.30%	20.40%
Significant unobservable inputs (Level 3)
Assets
Total assets at fair value	$ 840	$ 762
Assets as a percent of assets measured at fair value	1.20%	1.10%
Liabilities
Liabilities as a percent of liabilities measured at fair value	80.30%	87.70%
Recurring
Assets
Counterparty and cash collateral netting	$ (6)	$ (9)
Liabilities
Counterparty and cash collateral netting	14	28
Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed income securities	3,079	2,918
Equity securities	6,032	5,247
Short-term investments	264	850
Other investments: Free-standing derivatives	0	0
Separate account assets	3,444	3,393
Other assets	0	0
Total assets at fair value	12,819	12,408
Liabilities
Contractholder funds: Derivatives embedded in life and annuity contracts	0	0
Other liabilities: Free-standing derivatives	(1)	(1)
Total liabilities at fair value	(1)	(1)
Recurring | Quoted prices in active markets for identical assets (Level 1) | U.S. government and agencies
Assets
Fixed income securities	3,079	2,918
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 - CDO
Assets
Fixed income securities	0	0
Recurring | Quoted prices in active markets for identical assets (Level 1) | ABS - consumer and other
Assets
Fixed income securities	0	0
Recurring | Quoted prices in active markets for identical assets (Level 1) | RMBS
Assets
Fixed income securities	0	0
Recurring | Quoted prices in active markets for identical assets (Level 1) | CMBS
Assets
Fixed income securities	0	0
Recurring | Quoted prices in active markets for identical assets (Level 1) | Redeemable preferred stock
Assets
Fixed income securities	0	0
Recurring | Significant other observable inputs (Level 2)
Assets
Fixed income securities	55,307	54,363
Equity securities	379	256
Short-term investments	1,660	3,423
Other investments: Free-standing derivatives	132	119
Separate account assets	0	0
Other assets	0	0
Total assets at fair value	57,478	58,161
Liabilities
Contractholder funds: Derivatives embedded in life and annuity contracts	0	0
Other liabilities: Free-standing derivatives	(83)	(68)
Total liabilities at fair value	(83)	(68)
Recurring | Significant other observable inputs (Level 2) | U.S. government and agencies
Assets
Fixed income securities	537	719
Recurring | Significant other observable inputs (Level 2) | Municipal
Assets
Fixed income securities	8,227	7,208
Recurring | Significant other observable inputs (Level 2) | Corporate - public
Assets
Fixed income securities	31,963	31,414
Recurring | Significant other observable inputs (Level 2) | Corporate - privately placed
Assets
Fixed income securities	11,731	11,846
Recurring | Significant other observable inputs (Level 2) | Foreign government
Assets
Fixed income securities	1,021	1,075
Recurring | Significant other observable inputs (Level 2) | ABS - CDO
Assets
Fixed income securities	480	650
Recurring | Significant other observable inputs (Level 2) | ABS - consumer and other
Assets
Fixed income securities	645	452
Recurring | Significant other observable inputs (Level 2) | RMBS
Assets
Fixed income securities	578	727
Recurring | Significant other observable inputs (Level 2) | CMBS
Assets
Fixed income securities	102	248
Recurring | Significant other observable inputs (Level 2) | Redeemable preferred stock
Assets
Fixed income securities	23	24
Recurring | Significant unobservable inputs (Level 3)
Assets
Fixed income securities	606	558
Equity securities	210	163
Short-term investments	20	15
Other investments: Free-standing derivatives	1	1
Separate account assets	0	0
Other assets	0	1
Total assets at fair value	837	738
Liabilities
Contractholder funds: Derivatives embedded in life and annuity contracts	(286)	(290)
Other liabilities: Free-standing derivatives	0	(3)
Total liabilities at fair value	(286)	(293)
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	101	125
Recurring | Significant unobservable inputs (Level 3) | Corporate - public
Assets
Fixed income securities	108	78
Recurring | Significant unobservable inputs (Level 3) | Corporate - privately placed
Assets
Fixed income securities	224	263
Recurring | Significant unobservable inputs (Level 3) | Foreign government
Assets
Fixed income securities	0	0
Recurring | Significant unobservable inputs (Level 3) | ABS - CDO
Assets
Fixed income securities	99	27
Recurring | Significant unobservable inputs (Level 3) | ABS - consumer and other
Assets
Fixed income securities	48	42
Recurring | Significant unobservable inputs (Level 3) | RMBS
Assets
Fixed income securities	0	1
Recurring | Significant unobservable inputs (Level 3) | CMBS
Assets
Fixed income securities	26	22
Recurring | Significant unobservable inputs (Level 3) | Redeemable preferred stock
Assets
Fixed income securities	0	0
Non-recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Total assets at fair value	0	0
Non-recurring | Significant other observable inputs (Level 2)
Assets
Total assets at fair value	0	0
Non-recurring | Significant unobservable inputs (Level 3)
Assets
Total assets at fair value	3	24
Fair value
Assets
Total assets at fair value	$ 71,131	$ 71,322
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	$ 58,992	$ 57,839
Equity securities	6,621	5,666
Short-term investments	1,944	4,288
Other investments: Free-standing derivatives	127	111
Separate account assets	3,444	3,393
Other assets	0	1
Total assets at fair value	71,128	71,298
Liabilities
Contractholder funds: Derivatives embedded in life and annuity contracts	(286)	(290)
Other liabilities: Free-standing derivatives	(70)	(44)
Total liabilities at fair value	(356)	(334)
Fair value | Recurring | U.S. government and agencies
Assets
Fixed income securities	3,616	3,637
Fair value | Recurring | Municipal
Assets
Fixed income securities	8,328	7,333
Fair value | Recurring | Corporate - public
Assets
Fixed income securities	32,071	31,492
Fair value | Recurring | Corporate - privately placed
Assets
Fixed income securities	11,955	12,109
Fair value | Recurring | Foreign government
Assets
Fixed income securities	1,021	1,075
Fair value | Recurring | ABS - CDO
Assets
Fixed income securities	579	677
Fair value | Recurring | ABS - consumer and other
Assets
Fixed income securities	693	494
Fair value | Recurring | RMBS
Assets
Fixed income securities	578	728
Fair value | Recurring | CMBS
Assets
Fixed income securities	128	270
Fair value | Recurring | Redeemable preferred stock
Assets
Fixed income securities	23	24
Fair value | Non-recurring
Assets
Total assets at fair value	$ 3	$ 24