Consolidating Financial Statements (Details) - Consolidating Statement of Income (Unaudited) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Consolidating Statement of Income
Premiums		$ 6,882	$ 6,523	$ 20,114	$ 19,057
Net investment income		646	588	1,844	1,796
Fee income		109	113	324	342
Net realized investment gains (losses)	[1]	29	61	54	146
Other revenues		57	40	150	110
Total revenues		7,723	7,325	22,486	21,451
Claims and claim adjustment expenses		4,655	4,806	13,513	13,125
Amortization of deferred acquisition costs		1,117	1,059	3,259	3,094
General and administrative expenses		1,059	1,045	3,234	3,086
Interest expense		86	95	265	276
Total claims and expenses		6,917	7,005	20,271	19,581
Income before income taxes		806	320	2,215	1,870
Income tax expense (benefit)		97	27	313	365
Net income		709	293	1,902	1,505
Total other-than-temporary impairment losses		0	(5)	(1)	(11)
Other-than-temporary impairment losses recognized in net realized investment gains (losses)			(5)		(12)
Other-than-temporary impairment losses recognized in net realized investment gains (losses)		0		(1)
Other-than-temporary impairment losses recognized in other comprehensive income				0
Other-than-temporary impairment gains recognized in other comprehensive income		0	0		1
Reportable Legal Entities [Member] | TPC [Member]
Consolidating Statement of Income
Premiums		4,715	4,466	13,760	13,039
Net investment income		444	405	1,268	1,218
Fee income		108	113	323	342
Net realized investment gains (losses)		13	6	33	(2)
Other revenues		29	21	69	75
Total revenues		5,309	5,011	15,453	14,672
Claims and claim adjustment expenses		3,149	3,191	9,123	8,794
Amortization of deferred acquisition costs		757	716	2,189	2,077
General and administrative expenses		728	723	2,223	2,163
Interest expense		12	12	36	36
Total claims and expenses		4,646	4,642	13,571	13,070
Income before income taxes		663	369	1,882	1,602
Income tax expense (benefit)		104	74	314	361
Net income		559	295	1,568	1,241
Total other-than-temporary impairment losses			(2)	(1)	(4)
Other-than-temporary impairment losses recognized in net realized investment gains (losses)			(2)		(5)
Other-than-temporary impairment losses recognized in net realized investment gains (losses)				(1)
Other-than-temporary impairment gains recognized in other comprehensive income					1
Reportable Legal Entities [Member] | Other Subsidiaries [Member]
Consolidating Statement of Income
Premiums		2,167	2,057	6,354	6,018
Net investment income		193	189	553	574
Fee income		1		1
Net realized investment gains (losses)		9	56	13	90
Other revenues		30	21	85	42
Total revenues		2,400	2,323	7,006	6,724
Claims and claim adjustment expenses		1,506	1,615	4,390	4,331
Amortization of deferred acquisition costs		360	343	1,070	1,017
General and administrative expenses		326	314	997	911
Total claims and expenses		2,192	2,272	6,457	6,259
Income before income taxes		208	51	549	465
Income tax expense (benefit)		36	17	89	125
Net income		172	34	460	340
Total other-than-temporary impairment losses			(3)		(7)
Other-than-temporary impairment losses recognized in net realized investment gains (losses)			(3)		(7)
Reportable Legal Entities [Member] | TRV [Member]
Consolidating Statement of Income
Net investment income		9	7	23	17
Net realized investment gains (losses)		7	(1)	8	58
Total revenues		16	6	31	75
General and administrative expenses		7	10	18	19
Interest expense		74	83	229	240
Total claims and expenses		81	93	247	259
Income before income taxes		(65)	(87)	(216)	(184)
Income tax expense (benefit)		(43)	(60)	(90)	(117)
Net income of subsidiaries		731	329	2,028	1,581
Net income		709	302	1,902	1,514
Eliminations [Member]
Consolidating Statement of Income
Net investment income			(13)		(13)
Other revenues		(2)	(2)	(4)	(7)
Total revenues		(2)	(15)	(4)	(20)
General and administrative expenses		(2)	(2)	(4)	(7)
Total claims and expenses		(2)	(2)	(4)	(7)
Income before income taxes			(13)		(13)
Income tax expense (benefit)			(4)		(4)
Net income of subsidiaries		(731)	(329)	(2,028)	(1,581)
Net income		$ (731)	$ (338)	$ (2,028)	$ (1,590)

[1] Total other-than-temporary impairment (OTTI) losses were $0 million and $(5) million for the three months ended September 30, 2018 and 2017, respectively, and $(1) million and $(11) million for the nine months ended September 30, 2018 and 2017, respectively. Of total OTTI, credit losses of $0 million and $(5) million for the three months ended September 30, 2018 and 2017, respectively, and $(1) million and $(12) million for the nine months ended September 30, 2018 and 2017, respectively, were recognized in net realized investment gains. In addition, unrealized gains (losses) from other changes in total OTTI of $0 million for each of the three months ended September 30, 2018 and 2017, respectively, and $0 million and $1 million for the nine months ended September 30, 2018 and 2017, respectively, were recognized in other comprehensive income (loss) as part of changes in net unrealized gains (losses) on investment securities having credit losses recognized in the consolidated statement of income.