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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, 2020	Dec. 31, 2019
Variable Interest Entity Primary Beneficiary [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Investments held-to-maturity, fair value	$ 574	$ 892
Variable interest entity notes	1,178	1,539
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]
Total assets	552	890
Long-term debt	2,289	2,228
Medium-term notes	571	570
Investment agreements	289	304
Total liabilities	3,985	4,238
Gross	(81)	(311)
Ceded	15	24
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	552	890
Variable interest entity notes	836	1,136
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]
Total assets	574	892
Long-term debt	749	1,073
Medium-term notes	363	396
Investment agreements	407	394
Total liabilities	2,315	3,016
Gross	861	556
Ceded	48	56
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	574	892
Variable interest entity notes	796	1,153
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]
Total assets	0	0
Long-term debt	0	0
Medium-term notes	0	0
Investment agreements	0	0
Total liabilities	0	0
Gross	0	0
Ceded	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]
Investments held-to-maturity, fair value	0	0
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]
Total assets	0	0
Long-term debt	749	1,073
Medium-term notes	0	0
Investment agreements	0	0
Total liabilities	972	1,334
Gross	0	0
Ceded	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]
Investments held-to-maturity, fair value	0	0
Variable interest entity notes	223	261
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	574	892
Long-term debt	0	0
Medium-term notes	363	396
Investment agreements	407	394
Total liabilities	1,343	1,682
Gross	861	556
Ceded	48	56
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	574	892
Variable interest entity notes	$ 573	$ 892

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Represents the Company's balance sheet carrying amount of the financial guarantee insurance contracts ceded to third party reinsurers. The balance sheet carrying amount comprises the prepaid reinsurance premiums and reinsurance recoverable on paid and unpaid losses

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