Investments (Details) - Investment Information, Losses - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	$ 1,640		$ 1,640		$ 19,036
Continuous unrealized loss position, less than 12 months, gross unrealized losses	12		12		399
Continuous unrealized loss position, 12 months or longer, fair value	1,742		1,742		12,579
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	21		21		450
Continuous unrealized loss position, total, fair value	3,382		3,382		31,615
Continuous unrealized loss position, total, gross unrealized losses	33		33		849
Impairment charges	0	$ 0	2	$ 1
Cumulative OTTI credit losses recognized for securities held	$ 17	$ 54	$ 17	$ 54
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	$ 54		$ 54		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	352		352		1,011
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	1		1		11
Continuous unrealized loss position, total, fair value	406		406		1,495
Continuous unrealized loss position, total, gross unrealized losses	1		1		16
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	413		413		5,241
Continuous unrealized loss position, less than 12 months, gross unrealized losses	3		3		82
Continuous unrealized loss position, 12 months or longer, fair value	13		13		3,298
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	0		0		125
Continuous unrealized loss position, total, fair value	426		426		8,539
Continuous unrealized loss position, total, gross unrealized losses	3		3		207
Debt securities issued by foreign governments [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	67		67		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	157		157		328
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	1		1		5
Continuous unrealized loss position, total, fair value	224		224		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	288		288		593
Continuous unrealized loss position, less than 12 months, gross unrealized losses	1		1		9
Continuous unrealized loss position, 12 months or longer, fair value	235		235		1,070
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	2		2		29
Continuous unrealized loss position, total, fair value	523		523		1,663
Continuous unrealized loss position, total, gross unrealized losses	3		3		38
All other corporate bonds [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	818		818		12,622
Continuous unrealized loss position, less than 12 months, gross unrealized losses	8		8		303
Continuous unrealized loss position, 12 months or longer, fair value	985		985		6,872
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	17		17		280
Continuous unrealized loss position, total, fair value	1,803		1,803		19,494
Continuous unrealized loss position, total, gross unrealized losses	$ 25		$ 25		$ 583