Schedule II (details) - Condensed Statements of Comprehensive Income - 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
Condensed 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)
Net changes in benefit plan assets and obligations									29	16	66
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]
Condensed 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