Investments - Securities in a gross unrealized loss position (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	$ 5,931	$ 6,989
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	126	167
Estimated fair value, 12 months or longer	504	584
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	18	29
Estimated fair value, total	6,435	7,573
Available-for-sale securities, continuous unrealized loss position, accumulated loss	144	196
Corporate and other bonds
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	1,915	2,615
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	29	61
Estimated fair value, 12 months or longer	134	254
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	8	15
Estimated fair value, total	2,049	2,869
Available-for-sale securities, continuous unrealized loss position, accumulated loss	37	76
States, municipalities and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	845	959
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	31	32
Estimated fair value, 12 months or longer	33	23
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	0	1
Estimated fair value, total	878	982
Available-for-sale securities, continuous unrealized loss position, accumulated loss	31	33
Residential mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	2,188	2,136
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	44	44
Estimated fair value, 12 months or longer	170	201
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	6	7
Estimated fair value, total	2,358	2,337
Available-for-sale securities, continuous unrealized loss position, accumulated loss	50	51
Commercial mortgage-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	590	756
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	15	22
Estimated fair value, 12 months or longer	139	69
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	4	2
Estimated fair value, total	729	825
Available-for-sale securities, continuous unrealized loss position, accumulated loss	19	24
Other asset-backed
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	256	398
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	4	5
Estimated fair value, 12 months or longer	28	24
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	0	0
Estimated fair value, total	284	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	3,034	3,290
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	63	71
Estimated fair value, 12 months or longer	337	294
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	10	9
Estimated fair value, total	3,371	3,584
Available-for-sale securities, continuous unrealized loss position, accumulated loss	73	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	31	5
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	0	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	31	5
Available-for-sale securities, continuous unrealized loss position, accumulated loss	0	0
Foreign government
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	89	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	89	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	5,914	6,977
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	124	167
Estimated fair value, 12 months or longer	504	571
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss	18	25
Estimated fair value, total	6,418	7,548
Available-for-sale securities, continuous unrealized loss position, accumulated loss	142	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	0
Estimated fair value, 12 months or longer	0
Estimated fair value, total	2
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	2	0
Estimated fair value, 12 months or longer	0	13
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss		4
Estimated fair value, total	15	25
Available-for-sale securities, continuous unrealized loss position, accumulated loss	2	$ 4
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Estimated fair value, less than 12 months	17
Available-for-sale securities, continuous unrealized loss position, less than 12 months, accumulated loss	2
Estimated fair value, 12 months or longer	0
Estimated fair value, total	17
Available-for-sale securities, continuous unrealized loss position, accumulated loss	$ 2