Investments - Securities in a gross unrealized loss position (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	$ 4,104	$ 6,989
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	70	167
Estimated fair value, 12 months or longer	412	584
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	16	29
Estimated fair value, total	4,516	7,573
Available-for-sale securities, continuous unrealized loss position, accumulated loss	86	196
Corporate and other bonds
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	1,216	2,615
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	21	61
Estimated fair value, 12 months or longer	91	254
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	5	15
Estimated fair value, total	1,307	2,869
Available-for-sale securities, continuous unrealized loss position, accumulated loss	26	76
States, municipalities and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	583	959
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	6	32
Estimated fair value, 12 months or longer	56	23
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	1	1
Estimated fair value, total	639	982
Available-for-sale securities, continuous unrealized loss position, accumulated loss	7	33
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	1,522	2,136
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	25	44
Estimated fair value, 12 months or longer	106	201
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	3	7
Estimated fair value, total	1,628	2,337
Available-for-sale securities, continuous unrealized loss position, accumulated loss	28	51
Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	378	756
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	6	22
Estimated fair value, 12 months or longer	138	69
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	7	2
Estimated fair value, total	516	825
Available-for-sale securities, continuous unrealized loss position, accumulated loss	13	24
Other asset-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	129	398
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	4	5
Estimated fair value, 12 months or longer	10	24
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	0	0
Estimated fair value, total	139	422
Available-for-sale securities, continuous unrealized loss position, accumulated loss	4	5
Total asset-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	2,029	3,290
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	35	71
Estimated fair value, 12 months or longer	254	294
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	10	9
Estimated fair value, total	2,283	3,584
Available-for-sale securities, continuous unrealized loss position, accumulated loss	45	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	67	5
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	3	0
Estimated fair value, 12 months or longer	6	0
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	0	0
Estimated fair value, total	73	5
Available-for-sale securities, continuous unrealized loss position, accumulated loss	3	0
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	191	108
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	4	3
Estimated fair value, 12 months or longer	5	0
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	0	0
Estimated fair value, total	196	108
Available-for-sale securities, continuous unrealized loss position, accumulated loss	4	3
Total fixed maturity securities available-for-sale
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	4,086	6,977
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	69	167
Estimated fair value, 12 months or longer	412	571
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	16	25
Estimated fair value, total	4,498	7,548
Available-for-sale securities, continuous unrealized loss position, accumulated loss	85	192
Equity securities available-for-sale -- Common stock
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	2
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	2
Available-for-sale securities, continuous unrealized loss position, accumulated loss	1
Equity securities available-for-sale -- Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	16	12
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	0	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	16	25
Available-for-sale securities, continuous unrealized loss position, accumulated loss	0	$ 4
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	18
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	18
Available-for-sale securities, continuous unrealized loss position, accumulated loss	$ 1