Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues	$ 2,574	$ 2,366	$ 5,109	$ 4,696
Net realized investment gains (losses)	(1)	50	11	86
Operating Segments | Specialty
Revenues	1,061	893	2,094	1,789
Operating Segments | Commercial
Revenues	918	871	1,818	1,724
Operating Segments | International
Revenues	262	218	512	427
Operating Segments | Life & Group revenues
Revenues	329	330	664	661
Operating Segments | Corporate & Other revenues
Revenues	5	5	11	10
Eliminations
Revenues	0	(1)	(1)	(1)
Operating segments and eliminations, excluding segment reconciling Items
Revenues	2,575	2,316	5,098	4,610
Management & Professional Liability | Operating Segments | Specialty
Revenues	627	620	1,251	1,258
Surety | Operating Segments | Specialty
Revenues	145	136	274	260
Warranty & Alternative Risks | Operating Segments | Specialty
Revenues	289	137	569	271
Middle Market | Operating Segments | Commercial
Revenues	521	469	1,025	937
Small Business | Operating Segments | Commercial
Revenues	120	126	239	222
Other Commercial Insurance | Operating Segments | Commercial
Revenues	277	276	554	565
Canada | Operating Segments | International
Revenues	63	52	121	103
CNA Europe | Operating Segments | International
Revenues	91	79	179	151
Hardy | Operating Segments | International
Revenues	$ 108	$ 87	$ 212	$ 173