Short-Term and Long-Term Debt (Narrative) (Details) - USD ($)		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 4,496,000,000
Five-year revolving credit facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity		2,500,000,000
Borrowing capacity available to reimburse the banks for drawn LOCs		1,750,000,000
Minimum consolidated net worth		$ 10,500,000,000
Percentage of aggregate net proceeds of equity issuances		50.00%
Debt to capital ratio (low end of range)		0.35%
Debt to capital ratio (high end of range)		1.00%
LOC facility due August 2031 [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	[1]	$ 990,000,000
LOC facility due October 2031 [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	[1]	1,006,000,000
142 bps Surplus Note , Due 2023 [Member]
Debt Instrument [Line Items]
Loss on early extinguishment of debt			$ 5,000,000
Principal balance			240,000,000
9.76% Surplus Note, Due 2024 [Member]
Debt Instrument [Line Items]
Principal balance		50,000,000
Surplus notes		$ 50,000,000	50,000,000
Maturity date		Sep. 30, 2024
Interest rate		9.76%
6.56% Surplus Note, Due 2028 [Member]
Debt Instrument [Line Items]
Principal balance		$ 500,000,000
Surplus notes		$ 500,000,000	500,000,000
Maturity date		Mar. 31, 2028
Interest rate		6.56%
Capital surplus repayment threshold		$ 2,300,000,000
111 bps Surplus Note, Due 2028 [Member]
Debt Instrument [Line Items]
Principal balance		71,000,000
Surplus notes		$ 71,000,000	71,000,000
Maturity date		Sep. 24, 2028
226 bps Surplus Note, Due 2028 [Member]
Debt Instrument [Line Items]
Principal balance		$ 287,000,000
Surplus notes		$ 600,000,000	573,000,000
Maturity date		Oct. 01, 2028
6.03% Surplus Note, Due 2028 [Member]
Debt Instrument [Line Items]
Principal balance		$ 750,000,000
Surplus notes		$ 750,000,000	750,000,000
Maturity date		Dec. 31, 2028
Interest rate		6.03%
Capital surplus repayment threshold		$ 2,400,000,000
200 bps Surplus Note, Due 2035 [Member]
Debt Instrument [Line Items]
Principal balance		30,000,000
Surplus notes		$ 30,000,000	30,000,000
Maturity date		Sep. 28, 2035
100 bps Surplus Note, Due 2037 [Member]
Debt Instrument [Line Items]
Principal balance		$ 375,000,000
Surplus notes		$ 312,000,000	325,000,000
Maturity date		Oct. 09, 2037
155 bps Surplus Note, Due 2037 [Member]
Debt Instrument [Line Items]
Principal balance		$ 25,000,000
Surplus notes		$ 25,000,000	25,000,000
Maturity date		Jun. 30, 2037
4.20% Surplus Note, Due 2037 [Member]
Debt Instrument [Line Items]
Principal balance		$ 50,000,000
Surplus notes		$ 50,000,000	$ 50,000,000
Maturity date		Jul. 01, 2037
Interest rate		4.20%
4.50% Surplus Note, Due 2038 [Member]
Debt Instrument [Line Items]
Principal balance		$ 13,000,000
Surplus notes		$ 13,000,000
Maturity date		Jun. 30, 2038
Interest rate		4.50%
London Interbank Offered Rate (LIBOR) [Member] | 111 bps Surplus Note, Due 2028 [Member]
Debt Instrument [Line Items]
Variable rate		1.11%
London Interbank Offered Rate (LIBOR) [Member] | 226 bps Surplus Note, Due 2028 [Member]
Debt Instrument [Line Items]
Variable rate		2.26%
London Interbank Offered Rate (LIBOR) [Member] | 200 bps Surplus Note, Due 2035 [Member]
Debt Instrument [Line Items]
Variable rate		2.00%
London Interbank Offered Rate (LIBOR) [Member] | 100 bps Surplus Note, Due 2037 [Member]
Debt Instrument [Line Items]
Variable rate		1.00%
London Interbank Offered Rate (LIBOR) [Member] | 155 bps Surplus Note, Due 2037 [Member]
Debt Instrument [Line Items]
Variable rate		1.55%

[1] Our wholly-owned subsidiaries entered into irrevocable LOC facility agreements with third-party lenders supporting inter-company reinsurance agreements