Fair Value Of Financial Instruments (Fair Value Hierarchy Table Presents The Company's Assets And Liabilities At Fair Value Not Recorded On The Company's Consolidated Balance Sheet) (Detail) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Variable Interest Entity Primary Beneficiary [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Variable interest entity notes	$ 450	$ 623
Carrying Reported Amount Fair Value Disclosure [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Long-term debt	2,283	2,229
Medium-term notes	532	598
Investment agreements	269	269
Total liabilities	3,213	3,369
Gross	(235)	(282)
Ceded recoverable (liability)	(20)	(17)
Carrying Reported Amount Fair Value Disclosure [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Variable interest entity notes	129	273
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Long-term debt	595	631
Medium-term notes	328	396
Investment agreements	358	376
Total liabilities	1,414	1,679
Gross	921	811
Ceded recoverable (liability)	47	45
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Variable interest entity notes	133	276
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Inputs Level 1 [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Long-term debt	0	0
Medium-term notes	0	0
Investment agreements	0	0
Total liabilities	0	0
Gross	0	0
Ceded recoverable (liability)	0	0
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Inputs Level 1 [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Variable interest entity notes	0	0
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Inputs Level 2 [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Long-term debt	595	631
Medium-term notes	0	0
Investment agreements	0	0
Total liabilities	728	907
Gross	0	0
Ceded recoverable (liability)	0	0
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Inputs Level 2 [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Variable interest entity notes	133	276
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Inputs Level 3 [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Long-term debt	0	0
Medium-term notes	328	396
Investment agreements	358	376
Total liabilities	686	772
Gross	921	811
Ceded recoverable (liability)	47	45
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Inputs Level 3 [Member] | Variable Interest Entity Primary Beneficiary [Member] | Fair Value Measurements Nonrecurring [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Variable interest entity notes	$ 0	$ 0