Investments - Securities in a gross unrealized loss position (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	$ 4,600	$ 6,989
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	91	167
Estimated fair value, 12 months or longer	356	584
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	14	29
Estimated fair value, total	4,956	7,573
Available-for-sale securities, continuous unrealized loss position, accumulated loss	105	196
Corporate and other bonds
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	1,314	2,615
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	26	61
Estimated fair value, 12 months or longer	52	254
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	3	15
Estimated fair value, total	1,366	2,869
Available-for-sale securities, continuous unrealized loss position, accumulated loss	29	76
States, municipalities and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	742	959
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	15	32
Estimated fair value, 12 months or longer	24	23
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	0	1
Estimated fair value, total	766	982
Available-for-sale securities, continuous unrealized loss position, accumulated loss	15	33
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	1,722	2,136
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	34	44
Estimated fair value, 12 months or longer	141	201
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	4	7
Estimated fair value, total	1,863	2,337
Available-for-sale securities, continuous unrealized loss position, accumulated loss	38	51
Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	473	756
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	8	22
Estimated fair value, 12 months or longer	125	69
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	6	2
Estimated fair value, total	598	825
Available-for-sale securities, continuous unrealized loss position, accumulated loss	14	24
Other asset-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	159	398
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	4	5
Estimated fair value, 12 months or longer	14	24
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	1	0
Estimated fair value, total	173	422
Available-for-sale securities, continuous unrealized loss position, accumulated loss	5	5
Total asset-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	2,354	3,290
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	46	71
Estimated fair value, 12 months or longer	280	294
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	11	9
Estimated fair value, total	2,634	3,584
Available-for-sale securities, continuous unrealized loss position, accumulated loss	57	80
U.S. Treasury and obligations of government-sponsored enterprises
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	65	5
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	2	0
Estimated fair value, 12 months or longer	0	0
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	0	0
Estimated fair value, total	65	5
Available-for-sale securities, continuous unrealized loss position, accumulated loss	2	0
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	109	108
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	1	3
Estimated fair value, 12 months or longer	0	0
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	0	0
Estimated fair value, total	109	108
Available-for-sale securities, continuous unrealized loss position, accumulated loss	1	3
Total fixed maturity securities available-for-sale
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	4,584	6,977
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	90	167
Estimated fair value, 12 months or longer	356	571
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	14	25
Estimated fair value, total	4,940	7,548
Available-for-sale securities, continuous unrealized loss position, accumulated loss	104	192
Equity securities available-for-sale -- Common stock
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	1
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	0
Estimated fair value, 12 months or longer	0
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	0
Estimated fair value, total	1
Available-for-sale securities, continuous unrealized loss position, accumulated loss	0
Equity securities available-for-sale -- Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	15	12
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	1	0
Estimated fair value, 12 months or longer	0	13
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	0	4
Estimated fair value, total	15	25
Available-for-sale securities, continuous unrealized loss position, accumulated loss	1	$ 4
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	16
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	1
Estimated fair value, 12 months or longer	0
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	0
Estimated fair value, total	16
Available-for-sale securities, continuous unrealized loss position, accumulated loss	$ 1