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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Amount of cash inflow from exercise of stock options granted under share-based compensation arrangement.

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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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Gross number of share options (or share units) granted during the period.

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Percentage of performance based award payouts.

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Share based compensation by share based payment award, options term.

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Stock-Based Compensation Plans (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	May 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for future grants (in shares)	3,400,000		3,400,000
Number of stock options granted			0	0
Stock-based compensation expense	$ 3.4	$ 2.8	$ 7.1	$ 5.7
Aggregate intrinsic value of options exercised			10.9	11.9
Cash received from option exercises			$ 9.7	$ 15.1
2015 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares approved for issuance (in shares)					5,485,000
2007 Plan and 2015 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share based payment award options term			7 years
2007 Plan and 2015 Plan \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			3 years
2007 Plan and 2015 Plan \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			5 years
Restricted Stock \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			1 year
Restricted Stock \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			5 years
LTIP Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			3 years
LTIP Awards \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance based award payouts			0.00%
Granted after 2014 \| LTIP Awards \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance based award payouts			150.00%
Granted prior to 2015 \| LTIP Awards \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of performance based award payouts			200.00%