Shareholders' Equity and Dividend Availability (Details) - Dividend Availability $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) companies $ / shares	Dec. 31, 2019 USD ($) companies $ / shares	Dec. 31, 2018 USD ($) companies $ / shares
Statutory accounting principles
Dividends received by TRV and its two non-insurance holding companies from their U.S. insurance subsidiaries	$ 2,000	$ 2,500	$ 2,300
Cash dividends paid	861	844	814
Insurance subsidiaries [Member]
Statutory accounting principles
Statutory net income of the Company's domestic and international insurance subsidiaries	2,980	2,740	$ 2,610
Statutory capital and surplus of the Company's domestic and international insurance subsidiaries	$ 22,180	$ 21,330
Travelers [Member]
Statutory accounting principles
Number of non-insurance holding companies underneath TRV | companies	2	2	2
Cash dividends declared per common share (in dollars per share) | $ / shares	$ 3.37	$ 3.23	$ 3.03
Cash dividends paid	$ 861	$ 844	$ 814
Connecticut Insurance Department [Member] | Insurance subsidiaries [Member]
Statutory accounting principles
Maximum amount of dividends available to be paid by subsidiaries to their parent without prior approval of the Connecticut Insurance Department	$ 2,730