Investments (Details) - Investment Information, Losses - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	$ 752,000,000		$ 19,036,000,000
Continuous unrealized loss position, less than 12 months, gross unrealized losses	7,000,000		399,000,000
Continuous unrealized loss position, 12 months or longer, fair value	14,730,000,000		12,579,000,000
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	197,000,000		450,000,000
Continuous unrealized loss position, total, fair value	15,482,000,000		31,615,000,000
Continuous unrealized loss position, total, gross unrealized losses	204,000,000		849,000,000
Impairment charges	1,000,000	$ 0
Cumulative OTTI credit losses recognized for securities held	$ 56,000,000	$ 75,000,000
Cumulative credit component of other-than-temporary impairments (OTTI) on fixed maturities recognized in the consolidated statement of income for which a portion of the OTTI was recognized in other comprehensive income (loss) for fixed maturities held, as a percentage of the fixed maturity portfolio on a pre-tax basis (less than)	1.00%	1.00%
Cumulative credit component of other-than-temporary impairments (OTTI) on fixed maturities recognized in the consolidated statement of income for which a portion of the OTTI was recognized in other comprehensive income (loss) for fixed maturities held, as a percentage of shareholders' equity on an after-tax basis (less than)	1.00%	1.00%
U.S. Treasury securities and obligations of U.S. government and government agencies and authorities [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	$ 51,000,000		484,000,000
Continuous unrealized loss position, less than 12 months, gross unrealized losses			5,000,000
Continuous unrealized loss position, 12 months or longer, fair value	1,056,000,000		1,011,000,000
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	6,000,000		11,000,000
Continuous unrealized loss position, total, fair value	1,107,000,000		1,495,000,000
Continuous unrealized loss position, total, gross unrealized losses	6,000,000		16,000,000
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	103,000,000		5,241,000,000
Continuous unrealized loss position, less than 12 months, gross unrealized losses			82,000,000
Continuous unrealized loss position, 12 months or longer, fair value	2,464,000,000		3,298,000,000
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	24,000,000		125,000,000
Continuous unrealized loss position, total, fair value	2,567,000,000		8,539,000,000
Continuous unrealized loss position, total, gross unrealized losses	24,000,000		207,000,000
Debt securities issued by foreign governments [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	15,000,000		96,000,000
Continuous unrealized loss position, 12 months or longer, fair value	255,000,000		328,000,000
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	2,000,000		5,000,000
Continuous unrealized loss position, total, fair value	270,000,000		424,000,000
Continuous unrealized loss position, total, gross unrealized losses	2,000,000		5,000,000
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	32,000,000		593,000,000
Continuous unrealized loss position, less than 12 months, gross unrealized losses			9,000,000
Continuous unrealized loss position, 12 months or longer, fair value	1,333,000,000		1,070,000,000
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	16,000,000		29,000,000
Continuous unrealized loss position, total, fair value	1,365,000,000		1,663,000,000
Continuous unrealized loss position, total, gross unrealized losses	16,000,000		38,000,000
All other corporate bonds [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	551,000,000		12,622,000,000
Continuous unrealized loss position, less than 12 months, gross unrealized losses	7,000,000		303,000,000
Continuous unrealized loss position, 12 months or longer, fair value	9,622,000,000		6,872,000,000
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	149,000,000		280,000,000
Continuous unrealized loss position, total, fair value	10,173,000,000		19,494,000,000
Continuous unrealized loss position, total, gross unrealized losses	$ 156,000,000		$ 583,000,000