Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) segment industry	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting Information
Earned premiums									$ 4,710	$ 4,480	$ 4,243
Total revenues	$ 1,312	$ 1,402	$ 1,371	$ 1,364	$ 1,263	$ 1,278	$ 1,316	$ 1,285	5,449	5,142	4,945
Other Income									5	7	8
Fees and Commissions									15	13	12
Net Investment Income									595	572	549
Total									124	70	133
Income before income taxes	128	$ 253	$ 166	$ 265	216	$ 243	$ 248	$ 174	812	881	721
Identifiable assets	20,386				18,888				$ 20,386	18,888
Number of Industries Operated In | industry									2
Number of Insurance Segments | segment									4
Life Insurance Segment
Segment Reporting Information
Earned premiums									$ 228	209	198
Identifiable assets	1,366				1,325				1,366	1,325
Investments Segment
Segment Reporting Information
Identifiable assets	15,569				14,485				15,569	14,485
Consolidated Property and Casualty Insurance Entity
Segment Reporting Information
Identifiable assets	2,967				2,717				2,967	2,717
Corporate and Other [Member]
Segment Reporting Information
Identifiable assets	$ 484				$ 361				484	361
Operating Segments | Commercial Insurance
Segment Reporting Information
Earned premiums									3,089	2,996	2,856
Total revenues									3,094	3,000	2,860
Fees and Commissions									5	4	4
Income before income taxes									184	345	146
Operating Segments | Commercial Insurance | Commercial Casualty Line
Segment Reporting Information
Earned premiums									1,050	1,010	938
Operating Segments | Commercial Insurance | Commercial Property Line
Segment Reporting Information
Earned premiums									867	815	728
Operating Segments | Commercial Insurance | Commercial Auto Line
Segment Reporting Information
Earned premiums									594	561	528
Operating Segments | Commercial Insurance | Workers' Compensation Line
Segment Reporting Information
Earned premiums									354	367	370
Operating Segments | Commercial Insurance | Other Commercial Lines
Segment Reporting Information
Earned premiums									224	243	292
Operating Segments | Personal Insurance
Segment Reporting Information
Earned premiums									1,161	1,097	1,041
Total revenues									1,165	1,100	1,043
Fees and Commissions									4	3	2
Income before income taxes									(12)	(12)	10
Operating Segments | Personal Insurance | Personal Auto
Segment Reporting Information
Earned premiums									543	506	476
Operating Segments | Personal Insurance | Home Owner Line
Segment Reporting Information
Earned premiums									486	463	443
Operating Segments | Personal Insurance | Other Personal Lines
Segment Reporting Information
Earned premiums									132	128	122
Operating Segments | Excess and Surplus Lines Insurance
Segment Reporting Information
Earned premiums									183	168	148
Total revenues									184	169	148
Fees and Commissions									1	1	0
Income before income taxes									62	51	30
Operating Segments | Life Insurance Segment
Segment Reporting Information
Earned premiums									228	209	198
Total revenues									233	214	204
Fees and Commissions									5	5	6
Income before income taxes									1	(2)	(5)
Operating Segments | Investments Segment
Segment Reporting Information
Total revenues									719	642	682
Net Investment Income									595	572	549
Total									124	70	133
Income before income taxes									629	556	599
Corporate, Non-Segment [Member]
Segment Reporting Information
Total revenues									54	17	8
Corporate, Non-Segment [Member] | Reinsurance assumed and other non segment [Member]
Segment Reporting Information
Earned premiums									49	10	0
Corporate, Non-Segment [Member] | Corporate and Other [Member]
Segment Reporting Information
Other Income									5	7	8
Income before income taxes									$ (52)	$ (57)	$ (59)