Consolidating Financial Statements (details) - Consolidating Statement of Comprehensive Income (Unaudited) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidating Statement of Comprehensive Income
Net income	$ 551	$ 293	$ 595	$ 617	$ 943	$ 716	$ 664	$ 691	$ 2,056	$ 3,014	$ 3,439
Changes in net unrealized gains on investment securities having no credit losses recognized in the consolidated statement of income									294	(883)	(1,020)
Changes in net unrealized gains on investment securities having credit losses recognized in the consolidated statement of income									8	21	(14)
Net changes in benefit plan assets and obligations									29	16	66
Net changes in unrealized foreign currency translation									191	(41)	(461)
Other comprehensive income (loss) before income taxes									522	(887)	(1,429)
Income tax expense (benefit)									110	(289)	(392)
Other comprehensive income (loss), net of taxes, before other comprehensive income (loss) of subsidiaries									412	(598)	(1,037)
Other comprehensive income (loss), net of taxes									412	(598)	(1,037)
Comprehensive income									2,468	2,416	2,402
Travelers [Member]
Consolidating Statement of Comprehensive Income
Net income									2,064	3,014	3,439
Changes in net unrealized gains on investment securities having no credit losses recognized in the consolidated statement of income									(44)	11	(3)
Net changes in benefit plan assets and obligations									22	(20)	64
Other comprehensive income (loss) before income taxes									(22)	(9)	61
Income tax expense (benefit)									2	(1)	21
Other comprehensive income (loss), net of taxes, before other comprehensive income (loss) of subsidiaries									(24)	(8)	40
Other comprehensive income (loss) of subsidiaries									436	(590)	(1,077)
Other comprehensive income (loss), net of taxes									412	(598)	(1,037)
Comprehensive income									2,476	2,416	2,402
Reportable Legal Entities [Member] | TPC [Member]
Consolidating Statement of Comprehensive Income
Net income									1,728	2,184	2,666
Changes in net unrealized gains on investment securities having no credit losses recognized in the consolidated statement of income									313	(696)	(610)
Changes in net unrealized gains on investment securities having credit losses recognized in the consolidated statement of income									6	11	(12)
Net changes in benefit plan assets and obligations									(1)	25	2
Net changes in unrealized foreign currency translation									83	73	(306)
Other comprehensive income (loss) before income taxes									401	(587)	(926)
Income tax expense (benefit)									98	(222)	(257)
Other comprehensive income (loss), net of taxes, before other comprehensive income (loss) of subsidiaries									303	(365)	(669)
Other comprehensive income (loss), net of taxes									303	(365)	(669)
Comprehensive income									2,031	1,819	1,997
Reportable Legal Entities [Member] | Other Subsidiaries [Member]
Consolidating Statement of Comprehensive Income
Net income									462	970	998
Changes in net unrealized gains on investment securities having no credit losses recognized in the consolidated statement of income									25	(198)	(407)
Changes in net unrealized gains on investment securities having credit losses recognized in the consolidated statement of income									2	10	(2)
Net changes in benefit plan assets and obligations									8	11
Net changes in unrealized foreign currency translation									108	(114)	(155)
Other comprehensive income (loss) before income taxes									143	(291)	(564)
Income tax expense (benefit)									10	(66)	(156)
Other comprehensive income (loss), net of taxes, before other comprehensive income (loss) of subsidiaries									133	(225)	(408)
Other comprehensive income (loss), net of taxes									133	(225)	(408)
Comprehensive income									595	745	590
Reportable Legal Entities [Member] | Travelers [Member]
Consolidating Statement of Comprehensive Income
Net income									2,064	3,014	3,439
Changes in net unrealized gains on investment securities having no credit losses recognized in the consolidated statement of income									(44)	11	(3)
Net changes in benefit plan assets and obligations									22	(20)	64
Other comprehensive income (loss) before income taxes									(22)	(9)	61
Income tax expense (benefit)									2	(1)	21
Other comprehensive income (loss), net of taxes, before other comprehensive income (loss) of subsidiaries									(24)	(8)	40
Other comprehensive income (loss) of subsidiaries									436	(590)	(1,077)
Other comprehensive income (loss), net of taxes									412	(598)	(1,037)
Comprehensive income									2,476	2,416	2,402
Eliminations [Member]
Consolidating Statement of Comprehensive Income
Net income									(2,198)	(3,154)	(3,664)
Other comprehensive income (loss) of subsidiaries									(436)	590	1,077
Other comprehensive income (loss), net of taxes									(436)	590	1,077
Comprehensive income									$ (2,634)	$ (2,564)	$ (2,587)