Fair Value Measurements - The carrying value and fair value of financial instruments (Details) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Financial assets:
Mortgage loans		$ 1,082	$ 1,125
Policy loans		181	184
Financial liabilities:
Long-term debt		1,301	1,301
Level 1
Financial assets:
Cash and cash equivalents		1,659	2,338
Mortgage loans		0	0
Policy loans		0	0
Total financial assets not accounted for at fair value		1,659	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		1,316	1,351
Total financial liabilities not accounted for at fair value		1,316	1,351
Level 3
Financial assets:
Cash and cash equivalents		0	0
Mortgage loans		1,068	1,119
Policy loans		181	184
Total financial assets not accounted for at fair value		1,249	1,303
Financial liabilities:
Annuity benefits accumulated	[1]	31,983	32,461
Long-term debt		3	3
Total financial liabilities not accounted for at fair value		31,986	32,464
Carrying Value
Financial assets:
Cash and cash equivalents		1,659	2,338
Mortgage loans		1,082	1,125
Policy loans		181	184
Total financial assets not accounted for at fair value		2,922	3,647
Financial liabilities:
Annuity benefits accumulated	[1]	33,692	33,110
Long-term debt		1,301	1,301
Total financial liabilities not accounted for at fair value		34,993	34,411
Fair Value
Financial assets:
Cash and cash equivalents		1,659	2,338
Mortgage loans		1,068	1,119
Policy loans		181	184
Total financial assets not accounted for at fair value		2,908	3,641
Financial liabilities:
Annuity benefits accumulated	[1]	31,983	32,461
Long-term debt		1,319	1,354
Total financial liabilities not accounted for at fair value		$ 33,302	$ 33,815

[1] Excludes $209 million and $206 million of life contingent annuities in the payout phase at March 31, 2018 and December 31, 2017, respectively.