Stock-Based Compensation - Additional Information (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options exercise prices as percentage of the market value on the date of the grant	100.00%
Total intrinsic value of options exercised	$ 2.0	$ 0	$ 4.3
Total aggregate intrinsic value of options	16.9
Total aggregate intrinsic value of option exercisable	11.0
Grant date fair value of options granted	$ 17.48	$ 15.03	$ 11.40
Compensation expenses for stock options	7.7us-gaap_StockOptionPlanExpense	8.6us-gaap_StockOptionPlanExpense	9.9us-gaap_StockOptionPlanExpense
Restricted Stock [Member] \| LTIP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Portion of restricted stock award vesting on anniversary	0.333
Eligible age of officer for LTIP restricted stock award	55 years
Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to stock	29.3
Weighted-average recognition period for unrecognized compensation expense related to stock (years)	2 years
Number of anniversaries of grant date upon which restricted stock units vest	3
Installments in which restricted stock units vest and are converted to common stock	3
Compensation expense	32.6	32.6	34.2
Total fair value of shares, vested	38.1	34.1	33.1
Restricted Stock Units [Member] \| AIP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount of annual incentive award payment participant can elect to defer	50.00%
Additional common stock units for election of deferred annual incentive award, percentage	25.00%
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options vesting period	3 years
Stock options term, years	10 years
Unrecognized compensation expense related to stock	0.5
Weighted-average recognition period for unrecognized compensation expense related to stock (years)	2 years 4 months 24 days
Restricted Stock Units and Stock Options [Member] \| AIP and LTIP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to stock	0.8
Weighted-average recognition period for unrecognized compensation expense related to stock (years)	1 year 8 months 12 days
Compensation expense	6.1	6.3	6.6
Total fair value of shares, vested	$ 6.8	$ 5.7	$ 6.0

Additional common stock units for election of deferred annual incentive award, percentage.

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Age for eligible employee.

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Amount of annual incentive award payment participant can elect to defer.

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Fraction of restricted stock award vesting on anniversary.

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Installments in which restricted stock units vest and are converted to common stock.

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Number of anniversaries of grant date upon which restricted stock units vest.

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- Definition

Stock option exercise price as percentage of market value.

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The period in which a stock option can be exercised.

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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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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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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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- 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.

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- Definition

Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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- Definition

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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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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- Definition

Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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- Definition

The noncash expense that accounts for the value of stock or unit options distributed to employees as compensation.

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