Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) segment industry	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Segment Reporting Information
Earned premiums									$ 4,480	$ 4,243	$ 3,902
Total revenues	$ 1,263	$ 1,278	$ 1,316	$ 1,285	$ 1,262	$ 1,280	$ 1,214	$ 1,189	5,142	4,945	4,531
Other Income									7	8	9
Fees and Commissions									13	12	8
Net Investment Income									572	549	529
Total									70	133	83
Income before income taxes	216	$ 243	$ 248	$ 174	236	$ 259	$ 107	$ 119	881	721	714
Identifiable assets	18,888				18,748				$ 18,888	18,748
Number of Industries Operated In | industry									2
Number of Insurance Segments | segment									4
Life Insurance Segment
Segment Reporting Information
Earned premiums									$ 209	198	189
Identifiable assets	1,325				1,316				1,325	1,316
Investments Segment
Segment Reporting Information
Identifiable assets	14,485				14,441				14,485	14,441
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information
Identifiable assets	2,717				2,656				2,717	2,656
Corporate and Other Non-Segment
Segment Reporting Information
Other Income									17	8	9
Income before income taxes									(57)	(59)	(60)
Identifiable assets	$ 361				$ 335				361	335
Operating Segments | Commercial Insurance
Segment Reporting Information
Earned premiums									2,996	2,856	2,636
Total revenues									3,000	2,860	2,639
Fees and Commissions									4	4	3
Income before income taxes									345	146	186
Operating Segments | Commercial Insurance | Commercial Casualty Line
Segment Reporting Information
Earned premiums									1,010	938	856
Operating Segments | Commercial Insurance | Commercial Property Line
Segment Reporting Information
Earned premiums									815	728	623
Operating Segments | Commercial Insurance | Commercial Auto Line
Segment Reporting Information
Earned premiums									561	528	479
Operating Segments | Commercial Insurance | Workers' Compensation Line
Segment Reporting Information
Earned premiums									367	370	365
Operating Segments | Commercial Insurance | Other Commercial Lines
Segment Reporting Information
Earned premiums									243	292	313
Operating Segments | Personal Insurance
Segment Reporting Information
Earned premiums									1,097	1,041	961
Total revenues									1,100	1,043	962
Fees and Commissions									3	2	1
Income before income taxes									(12)	10	33
Operating Segments | Personal Insurance | Personal Auto
Segment Reporting Information
Earned premiums									506	476	443
Operating Segments | Personal Insurance | Home Owner Line
Segment Reporting Information
Earned premiums									463	443	403
Operating Segments | Personal Insurance | Other Personal Lines
Segment Reporting Information
Earned premiums									128	122	115
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information
Earned premiums									168	148	116
Total revenues									169	148	116
Fees and Commissions									1	0	0
Income before income taxes									51	30	14
Operating Segments | Life Insurance Segment
Segment Reporting Information
Earned premiums									209	198	189
Total revenues									214	204	193
Fees and Commissions									5	6	4
Income before income taxes									(2)	(5)	9
Operating Segments | Investments Segment
Segment Reporting Information
Total revenues									642	682	612
Net Investment Income									572	549	529
Total									70	133	83
Income before income taxes									$ 556	$ 599	$ 532