Shareholders' Equity and Dividend Availability (details) - Dividend Availability $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) companies $ / shares	Dec. 31, 2016 USD ($) companies $ / shares	Dec. 31, 2015 USD ($) companies $ / shares
Statutory accounting principles
Dividends received by TRV and its two non-insurance holding companies from their U.S. insurance subsidiaries	$ 2,330	$ 3,050	$ 3,750
Cash dividends declared per common share (in dollars per share) | $ / shares	$ 2.83	$ 2.62	$ 2.38
Cash dividends paid	$ 785	$ 757	$ 739
Insurance subsidiaries [Member]
Statutory accounting principles
Statutory net income of the Company's domestic and international insurance subsidiaries	2,300	3,200	$ 3,800
Statutory capital and surplus of the Company's domestic and international insurance subsidiaries	$ 20,450	$ 20,760
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	$ 2.83	$ 2.62	$ 2.38
Cash dividends paid	$ 785	$ 757	$ 739
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	$ 2,660