Investments (Details) - Investment Information, Losses - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	$ 530		$ 530		$ 19,036
Continuous unrealized loss position, less than 12 months, gross unrealized losses	2		2		399
Continuous unrealized loss position, 12 months or longer, fair value	4,431		4,431		12,579
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	38		38		450
Continuous unrealized loss position, total, fair value	4,961		4,961		31,615
Continuous unrealized loss position, total, gross unrealized losses	40		40		849
Impairment charges	1	$ 1	2	$ 1
Cumulative OTTI credit losses recognized for securities held	$ 54	$ 67	$ 54	$ 67
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%	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%	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	$ 15		$ 15		484
Continuous unrealized loss position, less than 12 months, gross unrealized losses	0		0		5
Continuous unrealized loss position, 12 months or longer, fair value	503		503		1,011
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	2		2		11
Continuous unrealized loss position, total, fair value	518		518		1,495
Continuous unrealized loss position, total, gross unrealized losses	2		2		16
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	126		126		5,241
Continuous unrealized loss position, less than 12 months, gross unrealized losses	0		0		82
Continuous unrealized loss position, 12 months or longer, fair value	232		232		3,298
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	1		1		125
Continuous unrealized loss position, total, fair value	358		358		8,539
Continuous unrealized loss position, total, gross unrealized losses	1		1		207
Debt securities issued by foreign governments [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	24		24		96
Continuous unrealized loss position, less than 12 months, gross unrealized losses	0		0		0
Continuous unrealized loss position, 12 months or longer, fair value	210		210		328
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	1		1		5
Continuous unrealized loss position, total, fair value	234		234		424
Continuous unrealized loss position, total, gross unrealized losses	1		1		5
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	88		88		593
Continuous unrealized loss position, less than 12 months, gross unrealized losses	0		0		9
Continuous unrealized loss position, 12 months or longer, fair value	472		472		1,070
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	4		4		29
Continuous unrealized loss position, total, fair value	560		560		1,663
Continuous unrealized loss position, total, gross unrealized losses	4		4		38
All other corporate bonds [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	277		277		12,622
Continuous unrealized loss position, less than 12 months, gross unrealized losses	2		2		303
Continuous unrealized loss position, 12 months or longer, fair value	3,014		3,014		6,872
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	30		30		280
Continuous unrealized loss position, total, fair value	3,291		3,291		19,494
Continuous unrealized loss position, total, gross unrealized losses	$ 32		$ 32		$ 583