Fair Value Measurements - The carrying value and fair value of financial instruments (Details) - USD ($) $ in Millions		Jun. 30, 2018	Dec. 31, 2017
Financial assets:
Mortgage loans		$ 1,147	$ 1,125
Policy loans		179	184
Financial liabilities:
Long-term debt		1,301	1,301
Level 1
Financial assets:
Cash and cash equivalents		1,810	2,338
Mortgage loans		0	0
Policy loans		0	0
Total financial assets not accounted for at fair value		1,810	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		1,262	1,351
Total financial liabilities not accounted for at fair value		1,262	1,351
Level 3
Financial assets:
Cash and cash equivalents		0	0
Mortgage loans		1,136	1,119
Policy loans		179	184
Total financial assets not accounted for at fair value		1,315	1,303
Financial liabilities:
Annuity benefits accumulated	[1]	33,204	32,461
Long-term debt		3	3
Total financial liabilities not accounted for at fair value		33,207	32,464
Carrying Value
Financial assets:
Cash and cash equivalents		1,810	2,338
Mortgage loans		1,147	1,125
Policy loans		179	184
Total financial assets not accounted for at fair value		3,136	3,647
Financial liabilities:
Annuity benefits accumulated	[1]	34,673	33,110
Long-term debt		1,301	1,301
Total financial liabilities not accounted for at fair value		35,974	34,411
Fair Value
Financial assets:
Cash and cash equivalents		1,810	2,338
Mortgage loans		1,136	1,119
Policy loans		179	184
Total financial assets not accounted for at fair value		3,125	3,641
Financial liabilities:
Annuity benefits accumulated	[1]	33,204	32,461
Long-term debt		1,265	1,354
Total financial liabilities not accounted for at fair value		$ 34,469	$ 33,815

[1] Excludes $213 million and $206 million of life contingent annuities in the payout phase at June 30, 2018 and December 31, 2017, respectively.