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Summary Of Significant Accounting Policies (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) state subsidiary independent_insurance_agency reporting_locations	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2008
Significant Accounting Policies
Unrealized gains reclassified to retained earnings for ASU 2016-01, after tax	$ 2,503,000,000
Excess tax benefit, amount	7,000,000
Income Tax Examination, Penalties Expense	$ 0
Number of operating subsidaries \| subsidiary	2
Number of independent insurance agencies \| independent_insurance_agency	1,702
Number of reporting locations \| reporting_locations	2,256
Number of states in which entity operates \| state	42
Defined Benefit Plan Qualifying Age				40 years
Current fair value of the BOLI invested assets and cash exceeded the current fair value of the contract holders' account value	$ 29,000,000
Other invested assets	103,000,000	$ 81,000,000
Depreciation expense	28,000,000	30,000,000	$ 36,000,000
Land, building and equipment impairments	0	0	0
Net cash used in financing activities	$ (614,000,000)	(426,000,000)	(498,000,000)
Minimum
Significant Accounting Policies
Estimated useful lives	3 years
Maximum
Significant Accounting Policies
Estimated useful lives	39 years 6 months
Life Policy Loans
Significant Accounting Policies
Other invested assets	$ 31,000,000	31,000,000
Venture Capital Funds
Significant Accounting Policies
Other invested assets	35,000,000	23,000,000
Real Estate Investment
Significant Accounting Policies
Other invested assets	$ 37,000,000	$ 27,000,000
10 Largest States
Significant Accounting Policies
Generated percentage of total earned premiums	59.70%	61.40%
Ohio, Largest State
Significant Accounting Policies
Generated percentage of total earned premiums	16.20%	16.80%
Indiana, Illinois, Pennsylvania, Georgia, Michigan and North Carolina \| Minimum
Significant Accounting Policies
Generated percentage of total earned premiums	5.00%
Indiana, Illinois, Pennsylvania, Georgia, Michigan and North Carolina \| Maximum
Significant Accounting Policies
Generated percentage of total earned premiums	7.00%
Largest Single Agency Relationship
Significant Accounting Policies
Generated percentage of total earned premiums	0.80%
All Other \| Maximum
Significant Accounting Policies
Generated percentage of total earned premiums	4.00%
Stock Options
Significant Accounting Policies
Weighted-average remaining contractual life for options expected to vest	10 years
Property Casualty Insurance Segment
Significant Accounting Policies
Number of operating subsidaries \| subsidiary	2
Stock Options
Significant Accounting Policies
Awards vesting period	3 years
Restricted Stock Units (RSUs)
Significant Accounting Policies
Awards vesting period	3 years
Accounting Standards Update 2016-09
Significant Accounting Policies
Net cash used in financing activities	$ 13,000,000	$ 12,000,000	11,000,000
Adjustment to reclassify certain tax effects from AOCI to retained earnings	0
New Accounting Pronouncement, Early Adoption, Effect \| Accumulated Other Comprehensive Income \| Accounting Standards Update 2018-02
Significant Accounting Policies
Adjustment to reclassify certain tax effects from AOCI to retained earnings	492,000,000	0	0
New Accounting Pronouncement, Early Adoption, Effect \| Retained Earnings \| Accounting Standards Update 2018-02
Significant Accounting Policies
Adjustment to reclassify certain tax effects from AOCI to retained earnings	$ (492,000,000)	$ 0	$ 0