Consolidating Financial Statements (Details) - Consolidating Statement of Income (Unaudited) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Consolidating Statement of Income
Premiums		$ 6,695	$ 6,351	$ 13,232	$ 12,534
Net investment income		595	598	1,198	1,208
Fee income		112	116	215	229
Net realized investment gains (losses)	[1]	36	80	25	85
Other revenues		39	39	93	70
Total revenues		7,477	7,184	14,763	14,126
Claims and claim adjustment expenses		4,562	4,225	8,858	8,319
Amortization of deferred acquisition costs		1,081	1,032	2,142	2,035
General and administrative expenses		1,113	1,045	2,175	2,041
Interest expense		90	92	179	181
Total claims and expenses		6,846	6,394	13,354	12,576
Income before income taxes		631	790	1,409	1,550
Income tax expense (benefit)		107	195	216	338
Net income		524	595	1,193	1,212
Total other-than-temporary impairment losses		(1)	(5)	(1)	(6)
Other-than-temporary impairment losses recognized in net realized investment gains (losses)			(5)		(7)
Other-than-temporary impairment losses recognized in net realized investment gains (losses)		(1)		(1)
Other-than-temporary impairment losses recognized in other comprehensive income		0		0
Other-than-temporary impairment gains recognized in other comprehensive income			0		1
Reportable Legal Entities [Member] | TPC [Member]
Consolidating Statement of Income
Premiums		4,577	4,345	9,045	8,573
Net investment income		412	401	824	813
Fee income		112	116	215	229
Net realized investment gains (losses)		18	(4)	20	(8)
Other revenues		13	30	40	54
Total revenues		5,132	4,888	10,144	9,661
Claims and claim adjustment expenses		3,064	2,851	5,974	5,603
Amortization of deferred acquisition costs		727	693	1,432	1,361
General and administrative expenses		766	737	1,495	1,440
Interest expense		13	12	24	24
Total claims and expenses		4,570	4,293	8,925	8,428
Income before income taxes		562	595	1,219	1,233
Income tax expense (benefit)		104	157	210	287
Net income		458	438	1,009	946
Total other-than-temporary impairment losses		(1)	(2)	(1)	(2)
Other-than-temporary impairment losses recognized in net realized investment gains (losses)			(2)		(3)
Other-than-temporary impairment losses recognized in net realized investment gains (losses)		(1)		(1)
Other-than-temporary impairment gains recognized in other comprehensive income					1
Reportable Legal Entities [Member] | Other Subsidiaries [Member]
Consolidating Statement of Income
Premiums		2,118	2,006	4,187	3,961
Net investment income		175	191	360	385
Net realized investment gains (losses)		16	25	4	34
Other revenues		27	12	55	21
Total revenues		2,336	2,234	4,606	4,401
Claims and claim adjustment expenses		1,498	1,374	2,884	2,716
Amortization of deferred acquisition costs		354	339	710	674
General and administrative expenses		343	305	671	597
Total claims and expenses		2,195	2,018	4,265	3,987
Income before income taxes		141	216	341	414
Income tax expense (benefit)		21	54	53	108
Net income		120	162	288	306
Total other-than-temporary impairment losses			(3)		(4)
Other-than-temporary impairment losses recognized in net realized investment gains (losses)			(3)		(4)
Reportable Legal Entities [Member] | TRV [Member]
Consolidating Statement of Income
Net investment income		8	6	14	10
Net realized investment gains (losses)		2	59	1	59
Total revenues		10	65	15	69
General and administrative expenses		5	6	11	9
Interest expense		77	80	155	157
Total claims and expenses		82	86	166	166
Income before income taxes		(72)	(21)	(151)	(97)
Income tax expense (benefit)		(18)	(16)	(47)	(57)
Net income of subsidiaries		578	600	1,297	1,252
Net income		524	595	1,193	1,212
Eliminations [Member]
Consolidating Statement of Income
Other revenues		(1)	(3)	(2)	(5)
Total revenues		(1)	(3)	(2)	(5)
General and administrative expenses		(1)	(3)	(2)	(5)
Total claims and expenses		(1)	(3)	(2)	(5)
Net income of subsidiaries		(578)	(600)	(1,297)	(1,252)
Net income		$ (578)	$ (600)	$ (1,297)	$ (1,252)

[1] Total other-than-temporary impairment (OTTI) losses were $(1) million and $(5) million for the three months ended June 30, 2018 and 2017, respectively, and $(1) million and $(6) million for the six months ended June 30, 2018 and 2017, respectively. Of total OTTI, credit losses of $(1) million and $(5) million for the three months ended June 30, 2018 and 2017, respectively, and $(1) million and $(7) million for the six months ended June 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 June 30, 2018 and 2017, respectively, and $0 million and $1 million for the six months ended June 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.