Investments - Securities in a Continuous Unrealized Loss Position (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair Value:
Less than 12 Months	$ 657	$ 1,085
12 Months or Longer	73	44
Total	730	1,129
Unrealized Losses:
Less than 12 Months	(11)	(53)
12 Months or Longer	(7)	(6)
Total	(18)	(59)
States and political subdivisions
Fair Value:
Less than 12 Months	107
12 Months or Longer	0
Total	107
Unrealized Losses:
Less than 12 Months	(1)
12 Months or Longer	0
Total	(1)
Corporate debt securities
Fair Value:
Less than 12 Months	410	747
12 Months or Longer	11	20
Total	421	767
Unrealized Losses:
Less than 12 Months	(4)	(24)
12 Months or Longer	(2)	(4)
Total	(6)	(28)
Foreign government bonds
Fair Value:
Less than 12 Months	85	106
12 Months or Longer	20	0
Total	105	106
Unrealized Losses:
Less than 12 Months	(4)	(13)
12 Months or Longer	(4)	0
Total	(8)	(13)
Preferred stock available for sale
Fair Value:
Less than 12 Months	55	140
12 Months or Longer	42	24
Total	97	164
Unrealized Losses:
Less than 12 Months	(2)	(4)
12 Months or Longer	(1)	(2)
Total	$ (3)	(6)
Equity securities available for sale
Fair Value:
Less than 12 Months		92
12 Months or Longer		0
Total		92
Unrealized Losses:
Less than 12 Months		(12)
12 Months or Longer		0
Total		$ (12)