Segment Information (details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Segment reporting information
Premiums		$ 5,981	$ 5,888
Net investment income		544	592
Fee income		117	114
Other revenues		53	25
Operating income (loss)		698	827
Net realized investment gains (losses)	[1]	(9)	10
Total revenues		6,686	6,629
Net realized investment gains (losses), net of tax		(7)	6
Net income		691	833
Reportable Segments [Member]
Segment reporting information
Premiums		5,981	5,888
Net investment income		544	592
Fee income		117	114
Other revenues		50	25
Total operating revenues		6,692	6,619
Operating income (loss)		759	891
Reportable Segments [Member] | Business and International Insurance [Member]
Segment reporting information
Premiums		3,599	3,620
Net investment income		415	454
Fee income		114	111
Other revenues		33	8
Total operating revenues		4,161	4,193
Operating income (loss)		476	515
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member]
Segment reporting information
Premiums		3,178	3,122
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Workers' compensation [Member]
Segment reporting information
Premiums		981	962
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Automobile [Member]
Segment reporting information
Premiums		491	468
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Commercial Property [Member]
Segment reporting information
Premiums		437	440
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | General Liability [Member]
Segment reporting information
Premiums		482	468
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Commercial multi-peril [Member]
Segment reporting information
Premiums		782	774
Reportable Segments [Member] | Business and International Insurance [Member] | Domestic [Member] | Other [Member]
Segment reporting information
Premiums		5	10
Reportable Segments [Member] | Business and International Insurance [Member] | International [Member]
Segment reporting information
Premiums		421	498
Reportable Segments [Member] | Bond & Specialty Insurance [Member]
Segment reporting information
Premiums		508	504
Net investment income		52	56
Other revenues		3	5
Total operating revenues		563	565
Operating income (loss)		144	124
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | General Liability [Member]
Segment reporting information
Premiums		234	236
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Fidelity and surety [Member]
Segment reporting information
Premiums		230	225
Reportable Segments [Member] | Bond & Specialty Insurance [Member] | Other [Member]
Segment reporting information
Premiums		44	43
Reportable Segments [Member] | Personal Insurance [Member]
Segment reporting information
Premiums		1,874	1,764
Net investment income		77	82
Fee income		3	3
Other revenues		14	12
Total operating revenues		1,968	1,861
Operating income (loss)		139	252
Reportable Segments [Member] | Personal Insurance [Member] | Automobile [Member]
Segment reporting information
Premiums		936	836
Reportable Segments [Member] | Personal Insurance [Member] | Homeowners and other [Member]
Segment reporting information
Premiums		938	928
Other [Member]
Segment reporting information
Other revenues		3
Operating income (loss)		(61)	(64)
After-tax interest expense		$ 59	$ 60

[1] Total other-than-temporary impairment (OTTI) losses were $(28) million and $(4) million for the three months ended March 31, 2016 and 2015, respectively. Of total OTTI, credit losses of $(18) million and $(3) million for the three months ended March 31, 2016 and 2015, respectively, were recognized in net realized investment gains (losses). In addition, unrealized losses from other changes in total OTTI of $(10) million and $(1) million for the three months ended March 31, 2016 and 2015, respectively, were recognized in other comprehensive income (loss) as part of changes in net unrealized gains on investment securities having credit losses recognized in the consolidated statement of income.