Schedule II (details) - Condensed Statements of Cash Flows - 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 Cash Flows
Net income	$ 551	$ 293	$ 595	$ 617	$ 943	$ 716	$ 664	$ 691	$ 2,056	$ 3,014	$ 3,439
Deferred federal income tax expense (benefit)									337	110	117
Other operating activities									(343)	189	56
Net cash provided by operating activities									3,762	4,202	3,434
Net sales (purchases) of short-term securities									(26)	(199)	(326)
Acquisition, net of cash acquired									(439)		(13)
Net cash provided by (used in) investing activities									(1,816)	(1,460)	317
Treasury stock acquired - share repurchase authorization									(1,378)	(2,400)	(3,150)
Treasury stock acquired - net employee share-based compensation									(62)	(72)	(74)
Dividends paid to shareholders									(785)	(757)	(739)
Payment of debt									(657)	(400)	(400)
Issuance of debt									789	491	392
Issuance of common stock - employee share options									173	332	183
Net cash used in financing activities									(1,920)	(2,806)	(3,733)
Net increase (decrease) in cash									37	(73)	6
Cash at beginning of year				307				380	307	380	374
Cash at end of year	$ 344				307				344	307	380
Cash received during the year for taxes									(514)	(892)	(1,207)
Cash paid during the year for interest									367	358	365
Travelers [Member]
Condensed Statement of Cash Flows
Net income									2,064	3,014	3,439
Equity in net income of subsidiaries									(2,190)	(3,154)	(3,664)
Dividends received from consolidated subsidiaries									2,289	2,998	3,833
Capital received from subsidiaries											3
Deferred federal income tax expense (benefit)									40	12	(6)
Change in income taxes payable									3	(48)	51
Other operating activities									(174)	73	113
Net cash provided by operating activities									2,032	2,895	3,769
Net sales (purchases) of short-term securities									397	(81)	(16)
Other investments, net									(34)	(8)	(20)
Acquisition, net of cash acquired									(477)
Net cash provided by (used in) investing activities									(114)	(89)	(36)
Treasury stock acquired - share repurchase authorization									(1,378)	(2,400)	(3,150)
Treasury stock acquired - net employee share-based compensation									(62)	(72)	(74)
Dividends paid to shareholders									(785)	(757)	(739)
Payment of debt									(657)	(400)	(400)
Issuance of debt									789	491	392
Issuance of common stock - employee share options									173	332	183
Other financing activities											55
Net cash used in financing activities									(1,920)	(2,806)	(3,733)
Net increase (decrease) in cash									(2)
Cash at beginning of year				$ 2				$ 2	2	2	2
Cash at end of year					$ 2					2	2
Cash received during the year for taxes									173	132	209
Cash paid during the year for interest									$ 320	$ 311	$ 318