Investments Investments (Details) - Investment Information, Losses - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	$ 27,078		$ 27,078		$ 9,132
Continuous unrealized loss position, less than 12 months, gross unrealized losses	605		605		68
Continuous unrealized loss position, 12 months or longer, fair value	6,124		6,124		5,780
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	275		275		110
Continuous unrealized loss position, total, fair value	33,202		33,202		14,912
Continuous unrealized loss position, total, gross unrealized losses	880		880		178
Impairment charges	1		1
Cumulative OTTI credit losses recognized for securities held	$ 67	$ 83	$ 67	$ 83
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%
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value					9,153
Continuous unrealized loss position, less than 12 months, gross unrealized losses					68
Continuous unrealized loss position, 12 months or longer, fair value					5,870
Continuous unrealized loss position, 12 months or longer, gross unrealized losses					117
Continuous unrealized loss position, total, fair value					15,023
Continuous unrealized loss position, total, gross unrealized losses					185
Impairment charges		$ 5		$ 7
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	$ 1,410		$ 1,410		1,150
Continuous unrealized loss position, less than 12 months, gross unrealized losses	20		20		5
Continuous unrealized loss position, 12 months or longer, fair value	458		458		470
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	3		3		3
Continuous unrealized loss position, total, fair value	1,868		1,868		1,620
Continuous unrealized loss position, total, gross unrealized losses	23		23		8
Obligations of states, municipalities and political subdivisions [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	6,321		6,321		505
Continuous unrealized loss position, less than 12 months, gross unrealized losses	101		101		2
Continuous unrealized loss position, 12 months or longer, fair value	2,856		2,856		2,959
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	136		136		47
Continuous unrealized loss position, total, fair value	9,177		9,177		3,464
Continuous unrealized loss position, total, gross unrealized losses	237		237		49
Debt securities issued by foreign governments [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	206		206		394
Continuous unrealized loss position, less than 12 months, gross unrealized losses	2		2		6
Continuous unrealized loss position, 12 months or longer, fair value	241		241		111
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	6		6		4
Continuous unrealized loss position, total, fair value	447		447		505
Continuous unrealized loss position, total, gross unrealized losses	8		8		10
Mortgage-backed securities, collateralized mortgage obligations and pass-through securities [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	1,557		1,557		1,021
Continuous unrealized loss position, less than 12 months, gross unrealized losses	41		41		7
Continuous unrealized loss position, 12 months or longer, fair value	256		256		250
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	8		8		4
Continuous unrealized loss position, total, fair value	1,813		1,813		1,271
Continuous unrealized loss position, total, gross unrealized losses	49		49		11
All other corporate bonds [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value	17,584		17,584		6,062
Continuous unrealized loss position, less than 12 months, gross unrealized losses	441		441		48
Continuous unrealized loss position, 12 months or longer, fair value	2,313		2,313		1,990
Continuous unrealized loss position, 12 months or longer, gross unrealized losses	122		122		52
Continuous unrealized loss position, total, fair value	19,897		19,897		8,052
Continuous unrealized loss position, total, gross unrealized losses	$ 563		$ 563		100
Equity securities [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value					21
Continuous unrealized loss position, 12 months or longer, fair value					90
Continuous unrealized loss position, 12 months or longer, gross unrealized losses					7
Continuous unrealized loss position, total, fair value					111
Continuous unrealized loss position, total, gross unrealized losses					7
Public common stock [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value					18
Continuous unrealized loss position, 12 months or longer, fair value					34
Continuous unrealized loss position, 12 months or longer, gross unrealized losses					1
Continuous unrealized loss position, total, fair value					52
Continuous unrealized loss position, total, gross unrealized losses					1
Non-redeemable preferred stock [Member]
Investment disclosure details
Continuous unrealized loss position, less than 12 months, fair value					3
Continuous unrealized loss position, 12 months or longer, fair value					56
Continuous unrealized loss position, 12 months or longer, gross unrealized losses					6
Continuous unrealized loss position, total, fair value					59
Continuous unrealized loss position, total, gross unrealized losses					$ 6