Fair Value Measurements - The carrying value and fair value of financial instruments (Details) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Financial assets:
Mortgage loans		$ 1,152	$ 1,125
Policy loans		176	184
Financial liabilities:
Long-term debt		1,302	1,301
Level 1
Financial assets:
Cash and cash equivalents		2,009	2,338
Mortgage loans		0	0
Policy loans		0	0
Total financial assets not accounted for at fair value		2,009	2,338
Financial liabilities:
Annuity benefits accumulated	[1]	0	0
Long-term debt		0	0
Total financial liabilities not accounted for at fair value		0	0
Level 2
Financial assets:
Cash and cash equivalents		0	0
Mortgage loans		0	0
Policy loans		0	0
Total financial assets not accounted for at fair value		0	0
Financial liabilities:
Annuity benefits accumulated	[1]	0	0
Long-term debt		1,257	1,351
Total financial liabilities not accounted for at fair value		1,257	1,351
Level 3
Financial assets:
Cash and cash equivalents		0	0
Mortgage loans		1,130	1,119
Policy loans		176	184
Total financial assets not accounted for at fair value		1,306	1,303
Financial liabilities:
Annuity benefits accumulated	[1]	33,923	32,461
Long-term debt		3	3
Total financial liabilities not accounted for at fair value		33,926	32,464
Carrying Value
Financial assets:
Cash and cash equivalents		2,009	2,338
Mortgage loans		1,152	1,125
Policy loans		176	184
Total financial assets not accounted for at fair value		3,337	3,647
Financial liabilities:
Annuity benefits accumulated	[1]	35,729	33,110
Long-term debt		1,302	1,301
Total financial liabilities not accounted for at fair value		37,031	34,411
Fair Value
Financial assets:
Cash and cash equivalents		2,009	2,338
Mortgage loans		1,130	1,119
Policy loans		176	184
Total financial assets not accounted for at fair value		3,315	3,641
Financial liabilities:
Annuity benefits accumulated	[1]	33,923	32,461
Long-term debt		1,260	1,354
Total financial liabilities not accounted for at fair value		$ 35,183	$ 33,815

[1] Excludes $229 million and $206 million of life contingent annuities in the payout phase at September 30, 2018 and December 31, 2017, respectively.