Schedule II (details) - Guarantees Footnote (details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Guarantor obligations
Amount of certain debt obligations of Travelers Property Casualty Corp. (TPC) and Travelers Insurance Group Holdings, Inc. (TIGHI) that are fully and unconditionally guaranteed by The Travelers Companies, Inc.	$ 700
7.75% Senior notes due April 15, 2026 [Member]
Guarantor obligations
Amount of certain debt obligations of Travelers Property Casualty Corp. (TPC) and Travelers Insurance Group Holdings, Inc. (TIGHI) that are fully and unconditionally guaranteed by The Travelers Companies, Inc.	$ 200	$ 200
Interest rate (percent)	7.75%	7.75%
6.375% Senior notes due March 15, 2033 [Member]
Guarantor obligations
Amount of certain debt obligations of Travelers Property Casualty Corp. (TPC) and Travelers Insurance Group Holdings, Inc. (TIGHI) that are fully and unconditionally guaranteed by The Travelers Companies, Inc.	$ 500	$ 500
Interest rate (percent)	6.375%	6.375%
Guarantees on payment of principal, premiums and interest on certain debt obligations [Member] | Travelers [Member] | 7.75% Senior notes due April 15, 2026 [Member]
Guarantor obligations
Amount of certain debt obligations of Travelers Property Casualty Corp. (TPC) and Travelers Insurance Group Holdings, Inc. (TIGHI) that are fully and unconditionally guaranteed by The Travelers Companies, Inc.	$ 200
Interest rate (percent)	7.75%
Guarantees on payment of principal, premiums and interest on certain debt obligations [Member] | Travelers [Member] | 6.375% Senior notes due March 15, 2033 [Member]
Guarantor obligations
Amount of certain debt obligations of Travelers Property Casualty Corp. (TPC) and Travelers Insurance Group Holdings, Inc. (TIGHI) that are fully and unconditionally guaranteed by The Travelers Companies, Inc.	$ 500
Interest rate (percent)	6.375%