Fair Value Measurements (details) - Changes in Level 3 - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Changes in Level 3 fair value category [Roll Forward]
Level 3 fair value category, beginning balance	$ 220	$ 256
Level 3 fair value category, total realized and unrealized investment gains (losses) reported in net realized investment gains	(1)	8
Level 3 fair value category, total unrealized investment gains (losses) reported in increases (decreases) in other comprehensive income (loss)	4	5
Level 3 fair value category purchases	312	123
Level 3 fair value category sales	(2)	(29)
Level 3 fair value category settlements/maturities	(47)	(66)
Gross transfers into Level 3	21	19
Gross transfers out of Level 3	(265)	(96)
Level 3 fair value category, ending balance	242	220
Level 3 fair value category, amount of total realized investment gains (losses) for the period included in the consolidated statement of income attributable to changes in the fair value of assets still held at the reporting date	(1)	(2)
Fixed maturities [Member]
Changes in Level 3 fair value category [Roll Forward]
Level 3 fair value category, beginning balance	184	218
Level 3 fair value category, total realized and unrealized investment gains (losses) reported in net realized investment gains		3
Level 3 fair value category, total unrealized investment gains (losses) reported in increases (decreases) in other comprehensive income (loss)	1	2
Level 3 fair value category purchases	312	123
Level 3 fair value category sales	(2)	(19)
Level 3 fair value category settlements/maturities	(47)	(66)
Gross transfers into Level 3	21	19
Gross transfers out of Level 3	(265)	(96)
Level 3 fair value category, ending balance	204	184
Other investments [Member]
Changes in Level 3 fair value category [Roll Forward]
Level 3 fair value category, beginning balance	36	38
Level 3 fair value category, total realized and unrealized investment gains (losses) reported in net realized investment gains	(1)	5
Level 3 fair value category, total unrealized investment gains (losses) reported in increases (decreases) in other comprehensive income (loss)	3	3
Level 3 fair value category sales		(10)
Level 3 fair value category, ending balance	38	36
Level 3 fair value category, amount of total realized investment gains (losses) for the period included in the consolidated statement of income attributable to changes in the fair value of assets still held at the reporting date	$ (1)	$ (2)