Investments- Portfolio Monitoring (Details 11) $ in Millions	Sep. 30, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Fixed income and equity securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	561	888
Fair value, continuous unrealized loss position for less than 12 months	$ 3,314	$ 5,270
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (58)	$ (145)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	109	188
Fair value, continuous unrealized loss position for 12 months or more	$ 285	$ 641
Unrealized losses, continuous unrealized loss position for 12 months or more	(45)	(90)
Total unrealized losses
Total unrealized losses	$ (103)	$ (235)
Fixed income securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	467	721
Fair value, continuous unrealized loss position for less than 12 months	$ 3,205	$ 5,070
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (48)	$ (126)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	79	126
Fair value, continuous unrealized loss position for 12 months or more	$ 256	$ 561
Unrealized losses, continuous unrealized loss position for 12 months or more	(39)	(74)
Total unrealized losses
Gross unrealized losses	$ (87)	$ (200)
U.S. government and agencies
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	13	6
Fair value, continuous unrealized loss position for less than 12 months	$ 146	$ 104
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (1)	$ (2)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	0	0
Fair value, continuous unrealized loss position for 12 months or more	$ 0	$ 0
Unrealized losses, continuous unrealized loss position for 12 months or more	0	0
Total unrealized losses
Gross unrealized losses	$ (1)	$ (2)
Municipal
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	6	8
Fair value, continuous unrealized loss position for less than 12 months	$ 26	$ 44
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (1)	$ (1)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	1	3
Fair value, continuous unrealized loss position for 12 months or more	$ 10	$ 18
Unrealized losses, continuous unrealized loss position for 12 months or more	(4)	(6)
Total unrealized losses
Gross unrealized losses	$ (5)	$ (7)
Corporate
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	367	629
Fair value, continuous unrealized loss position for less than 12 months	$ 2,870	$ 4,767
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (43)	$ (118)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	24	56
Fair value, continuous unrealized loss position for 12 months or more	$ 187	$ 414
Unrealized losses, continuous unrealized loss position for 12 months or more	(26)	(51)
Total unrealized losses
Gross unrealized losses	$ (69)	$ (169)
ABS
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	25	18
Fair value, continuous unrealized loss position for less than 12 months	$ 140	$ 95
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (1)	$ (1)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	8	13
Fair value, continuous unrealized loss position for 12 months or more	$ 18	$ 76
Unrealized losses, continuous unrealized loss position for 12 months or more	(6)	(9)
Total unrealized losses
Gross unrealized losses	$ (7)	$ (10)
RMBS
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	50	47
Fair value, continuous unrealized loss position for less than 12 months	$ 2	$ 3
Unrealized losses, continuous unrealized loss position for less than 12 months	$ 0	$ 0
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	43	50
Fair value, continuous unrealized loss position for 12 months or more	$ 27	$ 38
Unrealized losses, continuous unrealized loss position for 12 months or more	(1)	(3)
Total unrealized losses
Gross unrealized losses	$ (1)	$ (3)
CMBS
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	6	12
Fair value, continuous unrealized loss position for less than 12 months	$ 21	$ 57
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (2)	$ (4)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	3	4
Fair value, continuous unrealized loss position for 12 months or more	$ 14	$ 15
Unrealized losses, continuous unrealized loss position for 12 months or more	(2)	(5)
Total unrealized losses
Gross unrealized losses	$ (4)	$ (9)
Redeemable preferred stock
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security		1
Fair value, continuous unrealized loss position for less than 12 months		$ 0
Unrealized losses, continuous unrealized loss position for less than 12 months		$ 0
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security		0
Fair value, continuous unrealized loss position for 12 months or more		$ 0
Unrealized losses, continuous unrealized loss position for 12 months or more		0
Total unrealized losses
Gross unrealized losses		$ 0
Equity securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	94	167
Fair value, continuous unrealized loss position for less than 12 months	$ 109	$ 200
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (10)	$ (19)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	30	62
Fair value, continuous unrealized loss position for 12 months or more	$ 29	$ 80
Unrealized losses, continuous unrealized loss position for 12 months or more	(6)	(16)
Total unrealized losses
Gross unrealized losses on equity securities	$ (16)	$ (35)
Investment grade fixed income securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	399	559
Fair value, continuous unrealized loss position for less than 12 months	$ 2,991	$ 4,348
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (39)	$ (100)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	53	75
Fair value, continuous unrealized loss position for 12 months or more	$ 162	$ 350
Unrealized losses, continuous unrealized loss position for 12 months or more	(24)	(47)
Total unrealized losses
Total unrealized losses	$ (63)	$ (147)
Below investment grade fixed income securities
Less than 12 months
Number of issues, continuous unrealized loss position for less than 12 months | security	68	162
Fair value, continuous unrealized loss position for less than 12 months	$ 214	$ 722
Unrealized losses, continuous unrealized loss position for less than 12 months	$ (9)	$ (26)
12 months or more
Number of issues, continuous unrealized loss position for 12 months or more (in Security) | security	26	51
Fair value, continuous unrealized loss position for 12 months or more	$ 94	$ 211
Unrealized losses, continuous unrealized loss position for 12 months or more	(15)	(27)
Total unrealized losses
Total unrealized losses	$ (24)	$ (53)