Consolidating Financial Statements (Details) - Consolidating Statement of Income (Unaudited) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Consolidating Statement of Income
Premiums		$ 6,537	$ 6,183
Net investment income		603	610
Fee income		103	113
Net realized investment gains (losses)	[1]	(11)	5
Other revenues		54	31
Total revenues		7,286	6,942
Claims and claim adjustment expenses		4,296	4,094
Amortization of deferred acquisition costs		1,061	1,003
General and administrative expenses		1,062	996
Interest expense		89	89
Total claims and expenses		6,508	6,182
Income before income taxes		778	760
Income tax expense (benefit)		109	143
Net income		669	617
Total other-than-temporary impairment losses		0	(1)
Other-than-temporary impairment losses recognized in net realized investment gains (losses)		0	(2)
Other-than-temporary impairment gains recognized in other comprehensive income		0	1
Reportable Legal Entities [Member] | TPC [Member]
Consolidating Statement of Income
Premiums		4,468	4,228
Net investment income		412	412
Fee income		103	113
Net realized investment gains (losses)		2	(4)
Other revenues		27	24
Total revenues		5,012	4,773
Claims and claim adjustment expenses		2,910	2,752
Amortization of deferred acquisition costs		705	668
General and administrative expenses		729	703
Interest expense		11	12
Total claims and expenses		4,355	4,135
Income before income taxes		657	638
Income tax expense (benefit)		106	130
Net income		551	508
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		2,069	1,955
Net investment income		185	194
Net realized investment gains (losses)		(12)	9
Other revenues		28	9
Total revenues		2,270	2,167
Claims and claim adjustment expenses		1,386	1,342
Amortization of deferred acquisition costs		356	335
General and administrative expenses		328	292
Total claims and expenses		2,070	1,969
Income before income taxes		200	198
Income tax expense (benefit)		32	54
Net income		168	144
Total other-than-temporary impairment losses			(1)
Other-than-temporary impairment losses recognized in net realized investment gains (losses)			(1)
Reportable Legal Entities [Member] | TRV [Member]
Consolidating Statement of Income
Net investment income		6	4
Net realized investment gains (losses)		(1)
Total revenues		5	4
General and administrative expenses		6	3
Interest expense		78	77
Total claims and expenses		84	80
Income before income taxes		(79)	(76)
Income tax expense (benefit)		(29)	(41)
Net income of subsidiaries		719	652
Net income		669	617
Eliminations [Member]
Consolidating Statement of Income
Other revenues		(1)	(2)
Total revenues		(1)	(2)
General and administrative expenses		(1)	(2)
Total claims and expenses		(1)	(2)
Net income of subsidiaries		(719)	(652)
Net income		$ (719)	$ (652)

[1] Total other-than-temporary impairment (OTTI) gains (losses) were $0 and $(1) million for the three months ended March 31, 2018 and 2017, respectively. Of total OTTI, credit losses of $0 and $(2) million for the three months ended March 31, 2018 and 2017, respectively, were recognized in net realized investment gains (losses). In addition, unrealized gains from other changes in total OTTI of $0 and $1 million for the three months ended March 31, 2018 and 2017, respectively, were recognized in other comprehensive income (loss) as part of changes in net unrealized gains on investment securities having credit losses recognized in the consolidated statement of income.