Fair Value Measurements - The carrying value and fair value of financial instruments (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Financial assets:
Mortgage loans	$ 461	$ 377
Long-term debt	1,963	1,963
Level 1
Financial assets:
Cash and cash equivalents	3,365	1,665
Mortgage loans	0	0
Total financial assets not accounted for at fair value	3,365	1,665
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
Total financial assets not accounted for at fair value	0	0
Long-term debt	2,291	2,322
Total financial liabilities not accounted for at fair value	2,291	2,322
Level 3
Financial assets:
Cash and cash equivalents	0	0
Mortgage loans	479	382
Total financial assets not accounted for at fair value	479	382
Long-term debt	3	3
Total financial liabilities not accounted for at fair value	3	3
Carrying Value
Financial assets:
Cash and cash equivalents	3,365	1,665
Mortgage loans	461	377
Total financial assets not accounted for at fair value	3,826	2,042
Long-term debt	1,963	1,963
Total financial liabilities not accounted for at fair value	1,963	1,963
Fair Value
Financial assets:
Cash and cash equivalents	3,365	1,665
Mortgage loans	479	382
Total financial assets not accounted for at fair value	3,844	2,047
Long-term debt	2,294	2,325
Total financial liabilities not accounted for at fair value	$ 2,294	$ 2,325