Fair Value of Assets and Liabilities - Assets and liabilities measured at fair value (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Fixed income securities	$ 23,261	$ 24,222
Equity securities (1)	1,614	1,511
Short-term investments	725	566
Other investments:
Other investments: Free-standing derivatives	1	1	$ 1
Separate account assets	3,422	3,373
Derivative asset, fair value	$ (3)	$ (6)
% of total assets at fair value	0.00%	0.00%
Liabilities
Other liabilities: Free-standing derivatives		$ (3)	$ (8)
Total liabilities at fair value, Counterparty and cash collateral netting	$ 1	$ 2
% of total liabilities at fair value	(0.30%)	(0.60%)
Fair value
Other investments:
Total assets at fair value	$ 29,137	$ 29,778
% of total assets at fair value	100.00%	100.00%
Liabilities
Total liabilities at fair value	$ (345)	$ (329)
% of total liabilities at fair value	100.00%	100.00%
Limited partnership interests		$ 9
U.S. government and agencies
Assets
Fixed income securities	$ 804	1,014
Municipal
Assets
Fixed income securities	2,273	2,274
Foreign government
Assets
Fixed income securities	299	332
RMBS
Assets
Fixed income securities	253	333
CMBS
Assets
Fixed income securities	97	241
Redeemable preferred stock
Assets
Fixed income securities	14	16
Quoted prices in active markets for identical assets (Level 1)
Other investments:
Total assets at fair value	$ 5,528	$ 5,590
% of total assets at fair value	19.00%	18.70%
Liabilities
Total liabilities at fair value	$ 0	$ 0
% of total liabilities at fair value	0.00%	0.00%
Significant other observable inputs (Level 2)
Other investments:
Total assets at fair value	$ 23,145	$ 23,668
% of total assets at fair value	79.40%	79.50%
Liabilities
Total liabilities at fair value	$ (62)	$ (39)
% of total liabilities at fair value	18.00%	11.90%
Significant unobservable inputs (Level 3)
Other investments:
Total assets at fair value	$ 467	$ 526
% of total assets at fair value	1.60%	1.80%
Liabilities
Total liabilities at fair value	$ (284)	$ (292)
% of total liabilities at fair value	82.30%	88.70%
Recurring basis
Other investments:
Counterparty and cash collateral netting	$ (3)	$ (6)
Separate account assets		0
Derivative asset, fair value		(6)
Liabilities
Counterparty and cash collateral netting	1	2
Recurring basis | Fair value
Assets
Fixed income securities	23,261	24,222
Equity securities (1)	1,614	1,511
Short-term investments	725	566
Other investments:
Other investments: Free-standing derivatives	115	96
Separate account assets	3,422	3,373
Other assets	0	1
Total assets at fair value		29,769
Liabilities
Other liabilities: Free-standing derivatives	(61)	(40)
Recurring basis | Contractholder funds: Derivatives embedded in life and annuity contracts | Fair value
Liabilities
Contractholder funds: Derivatives embedded in life and annuity contracts	(284)	(289)
Recurring basis | U.S. government and agencies | Fair value
Assets
Fixed income securities	804	1,014
Recurring basis | Municipal | Fair value
Assets
Fixed income securities	2,273	2,274
Recurring basis | Corporate - public | Fair value
Assets
Fixed income securities	13,217	13,522
Recurring basis | Corporate - privately placed | Fair value
Assets
Fixed income securities	5,919	6,159
Recurring basis | Foreign government | Fair value
Assets
Fixed income securities	299	332
Recurring basis | ABS - CDO | Fair value
Assets
Fixed income securities	48	129
Recurring basis | ABS - consumer and other | Fair value
Assets
Fixed income securities	337	202
Recurring basis | RMBS | Fair value
Assets
Fixed income securities	253	333
Recurring basis | CMBS | Fair value
Assets
Fixed income securities	97	241
Recurring basis | Redeemable preferred stock | Fair value
Assets
Fixed income securities	14	16
Recurring basis | Quoted prices in active markets for identical assets (Level 1)
Assets
Fixed income securities	488	619
Equity securities (1)	1,508	1,432
Short-term investments	110	166
Other investments:
Other investments: Free-standing derivatives	0	0
Separate account assets		3,373
Other assets	0	0
Total assets at fair value		5,590
Liabilities
Other liabilities: Free-standing derivatives	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Contractholder funds: Derivatives embedded in life and annuity contracts
Liabilities
Contractholder funds: Derivatives embedded in life and annuity contracts	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | U.S. government and agencies
Assets
Fixed income securities	488	619
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Municipal
Assets
Fixed income securities	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Corporate - public
Assets
Fixed income securities	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Corporate - privately placed
Assets
Fixed income securities	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Foreign government
Assets
Fixed income securities	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | ABS - CDO
Assets
Fixed income securities	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | ABS - consumer and other
Assets
Fixed income securities	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | RMBS
Assets
Fixed income securities	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | CMBS
Assets
Fixed income securities	0	0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Redeemable preferred stock
Assets
Fixed income securities	0	0
Recurring basis | Significant other observable inputs (Level 2)
Assets
Fixed income securities	22,397	23,164
Equity securities (1)	16	3
Short-term investments	615	400
Other investments:
Other investments: Free-standing derivatives	117	101
Separate account assets	0
Other assets	0	0
Total assets at fair value		23,668
Liabilities
Other liabilities: Free-standing derivatives	(62)	(39)
Recurring basis | Significant other observable inputs (Level 2) | Contractholder funds: Derivatives embedded in life and annuity contracts
Liabilities
Contractholder funds: Derivatives embedded in life and annuity contracts	0	0
Recurring basis | Significant other observable inputs (Level 2) | U.S. government and agencies
Assets
Fixed income securities	316	395
Recurring basis | Significant other observable inputs (Level 2) | Municipal
Assets
Fixed income securities	2,216	2,215
Recurring basis | Significant other observable inputs (Level 2) | Corporate - public
Assets
Fixed income securities	13,168	13,475
Recurring basis | Significant other observable inputs (Level 2) | Corporate - privately placed
Assets
Fixed income securities	5,699	5,895
Recurring basis | Significant other observable inputs (Level 2) | Foreign government
Assets
Fixed income securities	299	332
Recurring basis | Significant other observable inputs (Level 2) | ABS - CDO
Assets
Fixed income securities	38	102
Recurring basis | Significant other observable inputs (Level 2) | ABS - consumer and other
Assets
Fixed income securities	297	160
Recurring basis | Significant other observable inputs (Level 2) | RMBS
Assets
Fixed income securities	253	333
Recurring basis | Significant other observable inputs (Level 2) | CMBS
Assets
Fixed income securities	97	241
Recurring basis | Significant other observable inputs (Level 2) | Redeemable preferred stock
Assets
Fixed income securities	14	16
Recurring basis | Significant unobservable inputs (Level 3)
Assets
Fixed income securities	376	439
Equity securities (1)	90	76
Short-term investments	0	0
Other investments:
Other investments: Free-standing derivatives	1	1
Separate account assets	0	0
Other assets	0	1
Total assets at fair value		517
Liabilities
Other liabilities: Free-standing derivatives	0	(3)
Recurring basis | Significant unobservable inputs (Level 3) | Contractholder funds: Derivatives embedded in life and annuity contracts
Liabilities
Contractholder funds: Derivatives embedded in life and annuity contracts	(284)	(289)
Recurring basis | Significant unobservable inputs (Level 3) | U.S. government and agencies
Assets
Fixed income securities	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Municipal
Assets
Fixed income securities	57	59
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - public
Assets
Fixed income securities	49	47
Recurring basis | Significant unobservable inputs (Level 3) | Corporate - privately placed
Assets
Fixed income securities	220	264
Recurring basis | Significant unobservable inputs (Level 3) | Foreign government
Assets
Fixed income securities	0	0
Recurring basis | Significant unobservable inputs (Level 3) | ABS - CDO
Assets
Fixed income securities	10	27
Recurring basis | Significant unobservable inputs (Level 3) | ABS - consumer and other
Assets
Fixed income securities	40	42
Recurring basis | Significant unobservable inputs (Level 3) | RMBS
Assets
Fixed income securities	0	0
Recurring basis | Significant unobservable inputs (Level 3) | CMBS
Assets
Fixed income securities	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Redeemable preferred stock
Assets
Fixed income securities	$ 0	0
Non-recurring basis | Fair value
Other investments:
Total assets at fair value		9
Non-recurring basis | Quoted prices in active markets for identical assets (Level 1)
Other investments:
Total assets at fair value		0
Non-recurring basis | Significant other observable inputs (Level 2)
Other investments:
Total assets at fair value		0
Non-recurring basis | Significant unobservable inputs (Level 3)
Other investments:
Total assets at fair value		$ 9