Compensation Related Costs Share Based Payments Disclosure [Line Items]

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Outstanding, Maximum Authorized Number Of Shares Issuable If Performance Factors Achieved

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No definition available.

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Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Payout Percentage If Performance Criteria Not Met

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- Definition

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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- Definition

The compensation expense recognized during the period pertaining to the deferred compensation arrangement.

+ References

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- Definition

The carrying amount of the liability as of the balance sheet date to an individual under a deferred compensation arrangement. This amount may be the result of periodic accruals made over the period of active employment, or reflect termination benefits resulting contractual terms or a death benefit.

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Minimum period the individual is required to perform services to be fully vested under the deferred compensation arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

Unrecognized cost of unvested share-based compensation awards.

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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.

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Description of award terms as to how many shares or portion of an award are no longer contingent on satisfaction of either a service condition, market condition or a performance condition, thereby giving the employee the legal right to convert the award to shares, to sell the shares, and be entitled to the cash proceeds of such sale. For example, vesting may be expressed as being 25 percent of the shares under option on each anniversary of the grant date.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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- Definition

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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Deferred Compensation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Payments Disclosure [Line Items]
Description of vesting term for awards granted			Awards granted under the plan generally vest ratably over a period of up to four to five years beginning on the first anniversary of the grant date or in full on the fourth or fifth anniversary of the grant date.
Unrecognized restricted stock units compensation expense			$ 8.0
Unrecognized restricted stock units compensation expense, weighted average recognition period			230 days
Deferred compensation payable			$ 12.1	$ 14.9
Compensation expense from deferred compensation			$ 7.7	13.1	$ 4.8
Minimum
Share Based Payments Disclosure [Line Items]
Service period for deferred compensation payable			3 years
Maximum
Share Based Payments Disclosure [Line Items]
Service period for deferred compensation payable			5 years
Restricted stock units
Share Based Payments Disclosure [Line Items]
Compensation expense from the vesting of restricted stock units			$ 38.4	$ 40.2	$ 45.8
Weighted average grant date fair value for restricted stock units granted (usd per share)			$ 18.94	$ 20.87	$ 21.61
Unrecognized restricted stock units compensation expense			$ 59.6
Unrecognized restricted stock units compensation expense, weighted average recognition period			1 year 9 months 18 days
Number of awards granted in the period (shares)		1,811,132	2,741,343	2,227,659
Restricted stock units vested and issued as common stock (shares)		931,180	1,238,733	1,197,260
Restricted stock units \| Minimum
Share Based Payments Disclosure [Line Items]
Stock units granted, ratable vesting period			4 years
Restricted stock units \| Maximum
Share Based Payments Disclosure [Line Items]
Stock units granted, ratable vesting period			5 years
Performance-Based Restricted Stock Units
Share Based Payments Disclosure [Line Items]
Number of awards granted in the period (shares)					115,473
Restricted stock units vested and issued as common stock (shares)			62,336
Performance payout percentage, if performance thresholds not achieved			0.00%
Maximum number of awards issuable, if performance factors achieved (shares)					288,683
Subsequent event \| Restricted stock units
Share Based Payments Disclosure [Line Items]
Number of awards granted in the period (shares)	1,618,208
Restricted stock units vested and issued as common stock (shares)	1,219,545