Stock-based Compensation - Additional Information 1 (Detail) (USD $) In Millions, except 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]
Compensation expense recognized	$ 10.7	$ 12.8	$ 13.7
Performance multiplier, minimum	0.00%
Performance multiplier, maximum	300.00%
Restricted Stock
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation expense recognized	7.3	9.8	13.5
Grant date fair value of share units, per share	$ 20.24	$ 16.44	$ 15.49
Unrecognized compensation cost related to unvested restricted stock	6.8
Compensation cost expected to be recognized period		10 months 24 days
Fair value of restricted stock vested	10.0	7.0	6.7
Restricted Stock \| Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Amortized vesting period of restricted stock	1 year
Restricted Stock \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Amortized vesting period of restricted stock	4 years
Performance Share Units (PSU)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Compensation expense recognized	3.4	3.0	0.2
Performance Periods	The performance periods consist of one year, two years and three years, respectively
Performance share units, risk free interest rate	0.60%		0.40%
Performance share units, Minimum volatility	38.00%		29.00%
Performance share units, Maximum volatility	70.00%		70.00%
Number of units granted	188,958	0	254,133
Grant date fair value of performance share units, granted	3.7		5.4
Grant date fair value of share units, per unit	19.81		21.14
Performance multiplier assumed	1
Minimum final number of shares of common stock issuable based on performance multiplier	0
Maximum final number of shares of common stock issuable based on performance multiplier	874,128
Performance multiplier, minimum	0.00%
Performance multiplier, maximum	300.00%
Unrecognized expense, performance share units	$ 2.5
Term for Amortization of Performance Share Compensation cost	through December 31, 2016
Performance Share Units (PSU) \| Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Amortized vesting period of restricted stock	1 year
Performance Share Units (PSU) \| Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Amortized vesting period of restricted stock	3 years

Amortization term related to performance shares.

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Final Number Of Potential Performance Shares Issuable Maximum.

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Final Number Of Potential Performance Shares Issuable Minimum.

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Grant date fair value of performance share units.

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Number of performance share units granted.

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Performance periods.

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Performance Units Adjustment For Performance Multiplier Multiplier Assumed.

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Performance units adjustment for performance multiplier range maximum.

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Performance units adjustment for performance multiplier range minimum.

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- Definition

Share based compensation arrangement by share based payment award equity instruments other than options grant date fair value.

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- Definition

Share based compensation arrangement by share based payment award equity instruments other than options unrecognized compensation cost.

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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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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

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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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

The estimated measure of the maximum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The estimated measure of the minimum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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