Shareholders' Equity - Changes in AOCI by Component (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Changes in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance			$ 6,504
Amounts reclassified from AOCI to:
Net realized investment gains (losses)	$ 7	$ (2)	54	$ (5)
Net investment income	99	94	322	291
Interest expense	(24)	(26)	(73)	(77)
Tax (provision) benefit	(105)	(1)	(157)	(62)
Ending balance	6,878		6,878
Net Unrealized Gains (Losses) on Investments with no Other-Than-Temporary Impairment [Member]
Changes in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	228	426	171	260
Other comprehensive income (loss) before reclassifications	27	(33)	133	146
Amounts reclassified from AOCI to:
Total amount reclassified from AOCI, net of tax	(15)	(3)	(64)	(16)
Net current period other comprehensive income (loss)	12	(36)	69	130
Ending balance	240	390	240	390
Net Unrealized Gains (Losses) on Investments with no Other-Than-Temporary Impairment [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Amounts reclassified from AOCI to:
Net realized investment gains (losses)	(24)	(3)	(71)	(20)
Net investment income	0	0	(28)	(3)
Interest expense	0	0	0	0
Total before tax	(24)	(3)	(99)	(23)
Tax (provision) benefit	9	0	35	7
Net Unrealized Gains (Losses) on Investments with Other-Than-Temporary Impairment [Member]
Changes in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	107	(24)	10	(15)
Other comprehensive income (loss) before reclassifications	(15)	13	81	(10)
Amounts reclassified from AOCI to:
Total amount reclassified from AOCI, net of tax	11	3	12	17
Net current period other comprehensive income (loss)	(4)	16	93	7
Ending balance	103	(8)	103	(8)
Net Unrealized Gains (Losses) on Investments with Other-Than-Temporary Impairment [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Amounts reclassified from AOCI to:
Net realized investment gains (losses)	17	4	18	25
Net investment income	0	0	0	0
Interest expense	0	0	0	0
Total before tax	17	4	18	25
Tax (provision) benefit	(6)	(1)	(6)	(8)
Cumulative Translation Adjustment [Member]
Changes in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	(27)	(26)	(39)	(16)
Other comprehensive income (loss) before reclassifications	3	(5)	15	(15)
Amounts reclassified from AOCI to:
Total amount reclassified from AOCI, net of tax	0	0	0	0
Net current period other comprehensive income (loss)	3	(5)	15	(15)
Ending balance	(24)	(31)	(24)	(31)
Cumulative Translation Adjustment [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Amounts reclassified from AOCI to:
Net realized investment gains (losses)	0	0	0	0
Net investment income	0	0	0	0
Interest expense	0	0	0	0
Total before tax	0	0	0	0
Tax (provision) benefit	0	0	0	0
Cash Flow Hedge [Member]
Changes in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	7	7	7	8
Other comprehensive income (loss) before reclassifications	0	0	0	0
Amounts reclassified from AOCI to:
Total amount reclassified from AOCI, net of tax	0	0	0	(1)
Net current period other comprehensive income (loss)	0	0	0	(1)
Ending balance	7	7	7	7
Cash Flow Hedge [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Amounts reclassified from AOCI to:
Net realized investment gains (losses)	0	0	0	0
Net investment income	0	0	0	0
Interest expense	0	0	0	(1)
Total before tax	0	0	0	(1)
Tax (provision) benefit	0	0	0	0
Total Accumulated Other Comprehensive Income [Member]
Changes in Accumulated Other Comprehensive Income [Roll Forward]
Beginning balance	315	383	149	237
Other comprehensive income (loss) before reclassifications	15	(25)	229	121
Amounts reclassified from AOCI to:
Total amount reclassified from AOCI, net of tax	(4)	0	(52)	0
Net current period other comprehensive income (loss)	11	(25)	177	121
Ending balance	326	358	326	358
Total Accumulated Other Comprehensive Income [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Amounts reclassified from AOCI to:
Net realized investment gains (losses)	(7)	1	(53)	5
Net investment income	0	0	(28)	(3)
Interest expense	0	0	0	(1)
Total before tax	(7)	1	(81)	1
Tax (provision) benefit	$ 3	$ (1)	$ 29	$ (1)