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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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- Definition

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

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- Definition

share based compensation arrangement by share based payment award, number of shares transferred Number of shares transferred from one Equity Plan to Another

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- Definition

Company shares held in retirement plan.

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- Definition

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

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- Definition

Amount of cash outflow for pension benefit. Includes, but is not limited to, employer contribution to fund plan asset and payment to retiree. Excludes other postretirement benefit.

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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.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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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.

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- Definition

Number of shares authorized for issuance under share-based payment arrangement.

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Employee Benefit Plans - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) Person shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee Benefits Disclosure [Line Items]
Minimum employment period for benefits	30 days
Postemployment benefits liability \| $	$ 22.2	$ 20.2
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 \| $	$ 191.6	162.7	$ 130.5
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 \| $	$ 3.0	$ 3.2	$ 2.2
Executive Deferred Compensation Plan
Employee Benefits Disclosure [Line Items]
Common shares reserved for issuance under executive deferred compensation plan	11,100,000
2017 Directors Equity Incentive Plan \| Non Employee Director Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Shares authorized	500,000
Equity Incentive Plan Twenty Ten [Member]
Employee Benefits Disclosure [Line Items]
Shares authorized	0
share based compensation arrangement by share based payment award, number of shares transferred	4,000,000.0
Equity Incentive Plan Twenty Fifteen [Domain]
Employee Benefits Disclosure [Line Items]
Shares authorized	17,000,000.0
Performance Based Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Estimated Number Of Participants \| Person	40
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%	250.00%
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 \| 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 \| $	$ 120.8	$ 106.8	$ 97.3
Defined Contribution Pension Plan 401k \| Upper Limit
Employee Benefits Disclosure [Line Items]
Matching contribution to defined contribution pension plan	6.00%
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%
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
Common Stock [Member]
Employee Benefits Disclosure [Line Items]
Employee Stock Ownership Plan Shares held	23,000,000.0