Fair Value (Carrying amount and estimated fair value of financial instrument assets and liabilities which are not measured at fair value) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Assets
Mortgage loans	$ 1,043	$ 1,068
Liabilities
Long term debt	2,777	2,776
Carrying Amount
Assets
Mortgage loans	1,043	1,068
Liabilities
Long term debt	2,777	2,776
Estimated Fair Value
Assets
Mortgage loans	1,107	1,151
Liabilities
Long term debt	3,016	3,148
Level 1 | Estimated Fair Value
Assets
Mortgage loans	0	0
Liabilities
Long term debt	0	0
Level 2 | Estimated Fair Value
Assets
Mortgage loans	0	0
Liabilities
Long term debt	3,016	3,148
Level 3 | Estimated Fair Value
Assets
Mortgage loans	1,107	1,151
Liabilities
Long term debt	$ 0	$ 0