Consolidating Financial Statements (details) - Consolidating Statement of Cash Flows (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 Cash Flows
Net income	$ 551	$ 293	$ 595	$ 617	$ 943	$ 716	$ 664	$ 691	$ 2,056	$ 3,014	$ 3,439
Net adjustments to reconcile net income to net cash provided by operating activities									1,706	1,188	(5)
Net cash provided by operating activities									3,762	4,202	3,434
Proceeds from maturities of fixed maturities									8,750	8,975	11,116
Proceeds from sales of investments, fixed maturities									1,854	1,417	1,950
Proceeds from sales of investments, equity securities									765	92	59
Proceeds from sales of investments, real estate investments									23	69	31
Proceeds from sales of investments, other investments									857	839	713
Purchases of investments, fixed maturities									(12,250)	(11,609)	(12,090)
Purchases of investments, equity securities									(459)	(51)	(49)
Purchases of investments, real estate investments									(59)	(48)	(123)
Purchases of investments, other investments									(541)	(580)	(534)
Net sales (purchases) of short-term securities									(26)	(199)	(326)
Securities transactions in course of settlement									(47)	(21)	(113)
Acquisition, net of cash acquired									(439)		(13)
Other investing activities									(244)	(344)	(304)
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
Excess tax benefits from share-based payment arrangements											55
Net cash used in financing activities									(1,920)	(2,806)	(3,733)
Effect of exchange rate changes on cash									11	(9)	(12)
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
Income taxes paid (received)									514	892	1,207
Interest paid									367	358	365
Travelers [Member]
Consolidating Statement of Cash Flows
Net income									2,064	3,014	3,439
Net cash provided by operating activities									2,032	2,895	3,769
Net sales (purchases) of short-term securities									397	(81)	(16)
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
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
Income taxes paid (received)									(173)	(132)	(209)
Interest paid									320	311	318
Reportable Legal Entities [Member] | TPC [Member]
Consolidating Statement of Cash Flows
Net income									1,728	2,184	2,666
Net adjustments to reconcile net income to net cash provided by operating activities									1,250	1,085	(577)
Net cash provided by operating activities									2,978	3,269	2,089
Proceeds from maturities of fixed maturities									6,576	6,589	7,543
Proceeds from sales of investments, fixed maturities									1,007	768	1,227
Proceeds from sales of investments, equity securities									97	47	25
Proceeds from sales of investments, other investments									610	586	503
Purchases of investments, fixed maturities									(8,513)	(7,921)	(8,276)
Purchases of investments, equity securities									(68)	(6)	(3)
Purchases of investments, real estate investments									(1)	(1)	(1)
Purchases of investments, other investments									(444)	(453)	(423)
Net sales (purchases) of short-term securities									(303)	(501)	179
Securities transactions in course of settlement									(55)	12	(52)
Acquisition, net of cash acquired											(13)
Other investing activities									(247)	(334)	(343)
Net cash provided by (used in) investing activities									(1,341)	(1,214)	366
Dividends paid to parent company									(1,624)	(2,140)	(2,450)
Net cash used in financing activities									(1,624)	(2,140)	(2,450)
Effect of exchange rate changes on cash									3	1	(1)
Net increase (decrease) in cash									16	(84)	4
Cash at beginning of year				141				225	141	225	221
Cash at end of year	157				141				157	141	225
Income taxes paid (received)									481	737	1,032
Interest paid									47	47	47
Reportable Legal Entities [Member] | Other Subsidiaries [Member]
Consolidating Statement of Cash Flows
Net income									462	970	998
Net adjustments to reconcile net income to net cash provided by operating activities									565	66	414
Net cash provided by operating activities									1,027	1,036	1,412
Proceeds from maturities of fixed maturities									2,168	2,380	3,563
Proceeds from sales of investments, fixed maturities									846	647	723
Proceeds from sales of investments, equity securities									414	45	34
Proceeds from sales of investments, real estate investments									23	69	31
Proceeds from sales of investments, other investments									260	253	210
Purchases of investments, fixed maturities									(3,697)	(3,676)	(3,787)
Purchases of investments, equity securities									(133)	(42)	(43)
Purchases of investments, real estate investments									(58)	(47)	(122)
Purchases of investments, other investments									(97)	(127)	(111)
Net sales (purchases) of short-term securities									(120)	383	(489)
Securities transactions in course of settlement									5	(32)	(61)
Acquisition, net of cash acquired									25
Other investing activities									3	(10)	39
Net cash provided by (used in) investing activities									(361)	(157)	(13)
Issuance of debt									14
Dividends paid to parent company									(665)	(858)	(1,383)
Capital contributions, loans and other transactions between subsidiaries											(3)
Net cash used in financing activities									(651)	(858)	(1,386)
Effect of exchange rate changes on cash									8	(10)	(11)
Net increase (decrease) in cash									23	11	2
Cash at beginning of year				164				153	164	153	151
Cash at end of year	$ 187				164				187	164	153
Income taxes paid (received)									206	287	384
Reportable Legal Entities [Member] | Travelers [Member]
Consolidating Statement of Cash Flows
Net income									2,064	3,014	3,439
Net adjustments to reconcile net income to net cash provided by operating activities									(32)	(119)	330
Net cash provided by operating activities									2,032	2,895	3,769
Proceeds from maturities of fixed maturities									6	6	10
Proceeds from sales of investments, fixed maturities									1	2
Proceeds from sales of investments, equity securities									254
Purchases of investments, fixed maturities									(40)	(12)	(27)
Purchases of investments, equity securities									(258)	(3)	(3)
Net sales (purchases) of short-term securities									397	(81)	(16)
Securities transactions in course of settlement									3	(1)
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
Excess tax benefits from share-based payment arrangements											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
Income taxes paid (received)									(173)	(132)	(209)
Interest paid									320	311	318
Eliminations [Member]
Consolidating Statement of Cash Flows
Net income									(2,198)	(3,154)	(3,664)
Net adjustments to reconcile net income to net cash provided by operating activities									(77)	156	(172)
Net cash provided by operating activities									(2,275)	(2,998)	(3,836)
Proceeds from sales of investments, other investments									(13)
Acquisition, net of cash acquired									13
Issuance of debt									(14)
Dividends paid to parent company									2,289	2,998	3,833
Capital contributions, loans and other transactions between subsidiaries											3
Net cash used in financing activities									$ 2,275	$ 2,998	$ 3,836