Debt (Long-Term Debt) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term debt, gross		$ 2,229	$ 2,228
Debt issuance costs		10	12
6.400% Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 0	115
Long-term debt, interest rate		6.40%
Long-term debt, maturity year		2022
7.000% Debentures Due 2025 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 46	46
Long-term debt, interest rate		7.00%
Long-term debt, maturity year		2025
7.150% Debentures Due 2027 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 100	100
Long-term debt, interest rate		7.15%
Long-term debt, maturity year		2027
6.625% Debentures Due 2028 [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 141	141
Long-term debt, interest rate		6.625%
Long-term debt, maturity year		2028
5.700% Senior Notes Due 2034 [Member]
Debt Instrument [Line Items]
Long-term debt, gross	[1]	$ 21	21
Long-term debt, interest rate		5.70%
Long-term debt, maturity year		2034
Surplus Notes due 2033 [Member]
Debt Instrument [Line Items]
Surplus notes outstanding	[2]	$ 940	940
Long-term debt, maturity year		2033
Accrued Interest [Member]
Debt Instrument [Line Items]
Long-term debt, gross		$ 991	$ 877

[1] Callable anytime at the greater of par or the present value of the remaining scheduled payments of principal and interest.

[2] Contractual interest rate is based on three month LIBOR plus 11.26%.