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- Definition

Defined contribution plan matching percent of compensation.

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- Definition

Employee Benefits Disclosure [Line Items]

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- Definition

Estimated Number Of Participants

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- Definition

Retirement Plans, Minimum Employment Period For Benefits

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- Definition

Total pretax intrinsic value of options exercised and the fair value of restricted stock vested based on the actual stock price at time of exercise/vesting.

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No definition available.

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- Definition

Share Based Compensation Arrangement By Share Based Payment Award Percentage Of Shares Vested

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- Definition

Company shares held in retirement plan.

+ References

No definition available.

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- Definition

The maximum number of shares authorized for issuance under the deferred compensation arrangement as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

Amount of payment for pension benefits. Excludes other postretirement benefits.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
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- Definition

The carrying amount as of the balance sheet date of the obligations recognized for the various benefits provided to former or inactive employees, their beneficiaries, and covered dependents after employment but before retirement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 712
-SubTopic 10
-Section 25
-Paragraph 4
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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- Definition

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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Employee Benefit Plans - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Person shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Employee Benefits Disclosure [Line Items]
Minimum employment period for benefits	30 days
Employee Stock Ownership Plan Shares held	24,600,000
Postemployment benefits liability \| $	$ 19.5	$ 21.7
Compensation, Percentage	100.00%
Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Aggregate fair value of the restricted equity awards that vested during the period \| $	$ 130.5	77.0	$ 105.4
Non Employee Director Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Vesting period of grants to date	11 months
Total pretax intrinsic value of options exercised and restricted stock vested \| $	$ 2.2	$ 3.0	$ 2.2
Executive Deferred Compensation Plan
Employee Benefits Disclosure [Line Items]
Common shares reserved for issuance under executive deferred compensation plan	11,100,000
2010 Equity Incentive Plan \| Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Shares authorized	18,000,000.0
2015 Equity Incentive Plan \| Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Shares authorized	13,000,000.0
2003 Directors Equity Incentive Plan \| Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Shares authorized	0
2003 Directors Equity Incentive Plan \| Non Employee Director Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Shares authorized	0
2017 Directors Equity Incentive Plan \| Non Employee Director Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Shares authorized	500,000
Performance Based Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Estimated Number Of Participants \| Person	45
Performance Based Restricted Equity Awards \| Insurance Operating Results \| Lower Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Performance Based Restricted Equity Awards \| Insurance Operating Results \| Upper Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	250.00%	250.00%	250.00%	250.00%	250.00%	200.00%
Performance Based Restricted Equity Awards \| Investment Results \| Lower Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Performance Based Restricted Equity Awards \| Investment Results \| Upper Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	200.00%	200.00%	200.00%	200.00%	200.00%	200.00%
Performance Based Restricted Equity Awards \| Growth in percentage of auto policies bundled with other specified types of policies [Member] \| Lower Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount			100.00%
Performance Based Restricted Equity Awards \| Growth in percentage of auto policies bundled with other specified types of policies [Member] \| Upper Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount			200.00%
Performance Based Restricted Equity Awards \| Growth in percentage of auto policies bundled with other specified types of policies, performance objective not achieved
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount			0.00%
Performance Based Restricted Equity Awards \| Unit growth in a particular customer segment [Member] \| Lower Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	85.00%	85.00%
Performance Based Restricted Equity Awards \| Unit growth in a particular customer segment [Member] \| Upper Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	150.00%	150.00%
Performance Based Restricted Equity Awards \| Unit growth in a particular customer segment, performance objective not achieved
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	0.00%	0.00%
Defined Contribution Pension Plan 401k
Employee Benefits Disclosure [Line Items]
Matching contributions \| $	$ 97.3	$ 86.8	$ 79.1
Defined Contribution Pension Plan 401k \| Upper Limit
Employee Benefits Disclosure [Line Items]
Matching contribution to defined contribution pension plan	6.00%
Vesting tranche one \| Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Vesting period of grants to date	3 years
Vesting tranche two \| Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Vesting period of grants to date	4 years
Vesting tranche three \| Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Vesting period of grants to date	5 years
Cash [Member]
Employee Benefits Disclosure [Line Items]
Compensation, Percentage	40.00%
Non Employee Director Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Compensation, Percentage	60.00%