Share-Based Incentive Compensation (details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended		12 Months Ended
	Feb. 06, 2018	May 31, 2017	May 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of stock option activity
Stock options outstanding, beginning of year, number (in shares)				8,560,036
Stock options granted, number (in shares)				2,106,022
Stock options exercised, number (in shares)				(1,784,731)
Stock options forfeited or expired, number (in shares)				(168,860)
Stock options outstanding, end of year, number (in shares)				8,712,467	8,560,036
Stock options outstanding, beginning of year, weighted average exercise price (in dollars per share)				$ 87.36
Stock options granted, weighted average exercise price (in dollars per share)				118.78
Stock options exercised, weighted average exercise price (in dollars per share)				73.19
Stock options forfeited or expired, weighted average exercise price (in dollars per share)				108.25
Stock options outstanding, end of year, weighted average exercise price (in dollars per share)				$ 97.45	$ 87.36
Stock options outstanding, weighted average contractual life remaining (in years)				6 years 10 months 24 days
Stock options outstanding, aggregate intrinsic value				$ 333
Stock options vested at end of year, number (in shares)				5,530,589
Stock options vested at end of year, weighted average exercise price (in dollars per share)				$ 89.67
Stock options vested at end of year, weighted average contractual life remaining (in years)				6 years 1 month 6 days
Stock options vested at end of year, aggregate intrinsic value				$ 254
Stock options exercisable at end of year, number (in shares)				2,835,011
Stock options exercisable at end of year, weighted average exercise price (in dollars per share)				$ 70.33
Stock options exercisable at end of year, weighted average contractual life remaining (in years)				4 years 2 months 12 days
Stock options exercisable at end of year, aggregate intrinsic value				$ 185
Stock option grants [Member]
Assumptions used in estimating fair value of options
Expected term of stock options				6 years		6 years
Expected volatility of the Company's stock (percent)				16.50%		19.29%
Weighted average volatility (percent)				16.50%	16.79%	19.29%
Expected annual dividend per share (in dollars per share)				$ 2.68		$ 2.20
Risk-free rate				2.08%		1.31%
Stock options granted, weighted average grant-date fair value of options granted (per share)				$ 16.15	$ 13.29	$ 15.78
Total intrinsic value of options exercised during the year				$ 90	$ 167	$ 120
Stock option grants [Member] | Minimum [Member]
Assumptions used in estimating fair value of options
Expected term of stock options					5 years
Expected volatility of the Company's stock (percent)					15.14%
Expected annual dividend per share (in dollars per share)					$ 2.44
Risk-free rate					1.36%
Stock option grants [Member] | Maxiumum [Member]
Assumptions used in estimating fair value of options
Expected term of stock options					6 years
Expected volatility of the Company's stock (percent)					16.80%
Expected annual dividend per share (in dollars per share)					$ 2.68
Risk-free rate					2.23%
The Travelers Companies, Inc 2014 Stock Incentive Plan [Member]
Share-based compensation plan description
Number of shares of the Company's common stock originally authorized for grant under the 2014 Incentive Plan				10,000,000
Additional number of shares of the Company's common stock authorized for grant under the 2014 Incentive Plan		2,500,000	4,400,000
Number of shares of the Company's common stock authorized for grant under the 2014 Incentive Plan				16,900,000
The Travelers Companies, Inc 2014 Stock Incentive Plan [Member] | Stock option grants [Member]
Share-based compensation plan description
Option term				10 years
General vesting period				3 years
The Travelers Companies, Inc 2014 Stock Incentive Plan [Member] | Stock option grants [Member] | Subsequent Event [Member]
Assumptions used in estimating fair value of options
Stock options granted, weighted average grant-date fair value of options granted (per share)	$ 20.13
Summary of stock option activity
Stock options granted, number (in shares)	1,632,361
Stock options granted, weighted average exercise price (in dollars per share)	$ 140.85
The Travelers Companies, Inc 2014 Stock Incentive Plan [Member] | Director deferred stock awards [Member]
Share-based compensation plan description
Number of days prior to date of annual meeting in year following date of grant in which annual deferred stock awards vest in full				1 day
Number of months after termination of service as a director before distributions of deferred stock awards begin				6 months