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	Sep. 30, 2019	Dec. 31, 2018
Variable Interest Entity Primary Beneficiary [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Investments held-to-maturity, fair value	$ 977	$ 925
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]
Other investments		1
Total assets	890	891
Long-term debt	2,199	2,249
Medium-term notes	564	620
Investment agreements	310	311
Total liabilities	4,233	4,444
Gross	(485)	(43)
Ceded	21	35
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]
Investments held-to-maturity, fair value	890	890
Variable interest entity notes	1,160	1,264
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]
Other investments		1
Total assets	977	926
Long-term debt	1,092	1,101
Medium-term notes	400	422
Investment agreements	414	388
Total liabilities	3,173	3,214
Gross	1,236	993
Ceded	78	65
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]
Investments held-to-maturity, fair value	977	925
Variable interest entity notes	1,267	1,303
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]
Other investments		1
Total assets		1
Long-term debt	1,092	1,101
Total liabilities	1,382	1,479
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	290	378
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]
Total assets	977	925
Medium-term notes	400	422
Investment agreements	414	388
Total liabilities	1,791	1,735
Gross	1,236	993
Ceded	78	65
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]
Investments held-to-maturity, fair value	977	925
Variable interest entity notes	$ 977	$ 925