Business Segments (Income Statement Information) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net written premiums		$ 8,059	$ 7,656	$ 7,345
Operating revenues
Net earned premiums		7,649	7,428	7,312
Net investment income		1,935	2,118	1,817
Non-insurance warranty revenue		1,252	1,161	1,007
Other revenues		26	31	50
Total operating revenues		10,862	10,738	10,186
Claims, Benefits and Expenses
Net incurred claims and benefits		6,149	5,783	5,547
Policyholders’ dividends		21	23	25
Amortization of deferred acquisition costs		1,410	1,383	1,335
Non-insurance warranty expense		1,159	1,082	923
Other insurance related expenses		1,028	1,038	1,039
Other expenses		220	235	301
Total claims, benefits and expenses		9,987	9,544	9,170
Core income (loss) before income tax		875	1,194	1,016
Income tax (expense) benefit on core income (loss)		(140)	(215)	(171)
Core income (loss)		735	979	845
Net investment gains (losses)		(54)	29	(52)
Income tax (expense) benefit on net investment gains (losses)		9	(8)	14
Net investment gains (losses), after tax	$ (216)	(45)	21	(38)
Net deferred tax asset remeasurement				6
Net income (loss)		690	1,000	813
Operating Segments | Specialty
Segment Reporting Information [Line Items]
Net written premiums		3,040	2,848	2,744
Operating revenues
Net earned premiums		2,883	2,773	2,732
Net investment income		449	556	439
Non-insurance warranty revenue		1,252	1,161	1,007
Other revenues		1	1	2
Total operating revenues		4,585	4,491	4,180
Claims, Benefits and Expenses
Net incurred claims and benefits		1,792	1,595	1,526
Policyholders’ dividends		3	5	5
Amortization of deferred acquisition costs		621	610	599
Non-insurance warranty expense		1,159	1,082	923
Other insurance related expenses		280	292	279
Other expenses		50	48	46
Total claims, benefits and expenses		3,905	3,632	3,378
Core income (loss) before income tax		680	859	802
Income tax (expense) benefit on core income (loss)		(145)	(188)	(173)
Core income (loss)		535	671	629
Operating Segments | Commercial
Segment Reporting Information [Line Items]
Net written premiums		3,565	3,315	3,060
Operating revenues
Net earned premiums		3,323	3,162	3,050
Net investment income		565	654	500
Non-insurance warranty revenue		0	0	0
Other revenues		25	29	28
Total operating revenues		3,913	3,845	3,578
Claims, Benefits and Expenses
Net incurred claims and benefits		2,436	2,130	2,053
Policyholders’ dividends		18	18	20
Amortization of deferred acquisition costs		592	537	505
Non-insurance warranty expense		0	0	0
Other insurance related expenses		505	505	505
Other expenses		34	32	43
Total claims, benefits and expenses		3,585	3,222	3,126
Core income (loss) before income tax		328	623	452
Income tax (expense) benefit on core income (loss)		(67)	(134)	(95)
Core income (loss)		261	489	357
Operating Segments | International
Segment Reporting Information [Line Items]
Net written premiums		961	971	1,018
Operating revenues
Net earned premiums		940	974	1,001
Net investment income		58	63	57
Non-insurance warranty revenue		0	0	0
Other revenues		0	0	1
Total operating revenues		998	1,037	1,059
Claims, Benefits and Expenses
Net incurred claims and benefits		629	624	699
Policyholders’ dividends		0	0	0
Amortization of deferred acquisition costs		197	236	231
Non-insurance warranty expense		0	0	0
Other insurance related expenses		136	130	135
Other expenses		(7)	8	14
Total claims, benefits and expenses		955	998	1,079
Core income (loss) before income tax		43	39	(20)
Income tax (expense) benefit on core income (loss)		(5)	(9)	1
Core income (loss)		38	30	(19)
Operating Segments | Life & Group
Segment Reporting Information [Line Items]
Net written premiums		494	523	524
Operating revenues
Net earned premiums		504	520	530
Net investment income		851	820	801
Non-insurance warranty revenue		0	0	0
Other revenues		0	0	2
Total operating revenues		1,355	1,340	1,333
Claims, Benefits and Expenses
Net incurred claims and benefits		1,286	1,416	1,218
Policyholders’ dividends		0	0	0
Amortization of deferred acquisition costs		0	0	0
Non-insurance warranty expense		0	0	0
Other insurance related expenses		109	115	122
Other expenses		7	8	7
Total claims, benefits and expenses		1,402	1,539	1,347
Core income (loss) before income tax		(47)	(199)	(14)
Income tax (expense) benefit on core income (loss)		56	90	57
Core income (loss)		9	(109)	43
Operating Segments | Corporate & Other
Segment Reporting Information [Line Items]
Net written premiums		0	1	0
Operating revenues
Net earned premiums		0	1	0
Net investment income		12	25	20
Non-insurance warranty revenue		0	0	0
Other revenues		5	6	19
Total operating revenues		17	32	39
Claims, Benefits and Expenses
Net incurred claims and benefits		6	18	51
Policyholders’ dividends		0	0	0
Amortization of deferred acquisition costs		0	0	0
Non-insurance warranty expense		0	0	0
Other insurance related expenses		(1)	(2)	(1)
Other expenses		141	144	193
Total claims, benefits and expenses		146	160	243
Core income (loss) before income tax		(129)	(128)	(204)
Income tax (expense) benefit on core income (loss)		21	26	39
Core income (loss)		(108)	(102)	(165)
Eliminations
Segment Reporting Information [Line Items]
Net written premiums		(1)	(2)	(1)
Operating revenues
Net earned premiums		(1)	(2)	(1)
Net investment income		0	0	0
Non-insurance warranty revenue		0	0	0
Other revenues		(5)	(5)	(2)
Total operating revenues		(6)	(7)	(3)
Claims, Benefits and Expenses
Net incurred claims and benefits		0	0	0
Policyholders’ dividends		0	0	0
Amortization of deferred acquisition costs		0	0	0
Non-insurance warranty expense		0	0	0
Other insurance related expenses		(1)	(2)	(1)
Other expenses		(5)	(5)	(2)
Total claims, benefits and expenses		(6)	(7)	(3)
Core income (loss) before income tax		0	0	0
Income tax (expense) benefit on core income (loss)		0	0	0
Core income (loss)		$ 0	$ 0	$ 0