Fair Value Measurements - The carrying value and fair value of financial instruments (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Financial assets:
Mortgage loans	$ 1,482	$ 1,329
Policy loans	154	164
Financial liabilities:
Long-term debt	2,108	1,473
Level 1
Financial assets:
Cash and cash equivalents	3,747	2,314
Mortgage loans	0	0
Policy loans	0	0
Total financial assets not accounted for at fair value	3,747	2,314
Financial liabilities:
Annuity benefits accumulated	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	0	0
Long-term debt	2,394	1,619
Total financial liabilities not accounted for at fair value	2,394	1,619
Level 3
Financial assets:
Cash and cash equivalents	0	0
Mortgage loans	1,507	1,346
Policy loans	154	164
Total financial assets not accounted for at fair value	1,661	1,510
Financial liabilities:
Annuity benefits accumulated	42,938	40,182
Long-term debt	3	3
Total financial liabilities not accounted for at fair value	42,941	40,185
Carrying Value
Financial assets:
Cash and cash equivalents	3,747	2,314
Mortgage loans	1,482	1,329
Policy loans	154	164
Total financial assets not accounted for at fair value	5,383	3,807
Financial liabilities:
Annuity benefits accumulated	41,082	40,159
Long-term debt	2,108	1,473
Total financial liabilities not accounted for at fair value	43,190	41,632
Fair Value
Financial assets:
Cash and cash equivalents	3,747	2,314
Mortgage loans	1,507	1,346
Policy loans	154	164
Total financial assets not accounted for at fair value	5,408	3,824
Financial liabilities:
Annuity benefits accumulated	42,938	40,182
Long-term debt	2,397	1,622
Total financial liabilities not accounted for at fair value	$ 45,335	$ 41,804