Represents the percentage over which forfeiture and non-transferable restrictions generally lapse in year four.

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Represents the percentage over which forfeiture and non-transferable restrictions generally lapse in year one.

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Represents the percentage over which forfeiture and non-transferable restrictions generally lapse in year three.

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No definition available.

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Represents the percentage over which forfeiture and non-transferable restrictions generally lapse in year two.

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Represents the period over which forfeiture and non-transferable restrictions generally lapse.

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The period of time, from the grant date, after which the equity-based option award expires.

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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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Unrecognized cost of unvested share-based compensation awards.

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

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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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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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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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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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 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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Stock-Based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stock-based compensation
Compensation expense related to the Plan	$ 12.8	$ 9.8	$ 6.8
Restricted Stock
Stock-based compensation
Period over which forfeiture and non-transferable restrictions lapse	4 years
Percentage over which unrecognized compensation cost is expected to be recognized year one	15.00%
Percentage over which unrecognized compensation cost is expected to be recognized year two	15.00%
Percentage over which unrecognized compensation cost is expected to be recognized year three	20.00%
Percentage over which unrecognized compensation cost is expected to be recognized year four	50.00%
Unrecognized compensation cost related to the restricted stock	20.0
Shares
Balance at the beginning of the period (in shares)	1,168,200
Granted (in shares)	314,677	448,026	431,339
Vested (in shares)	(373,450)
Forfeited (in shares)	(7,042)
Balance at the end of the period (in shares)	1,102,385	1,168,200
Weighted Average Grant-Date Fair value
Balance at the beginning of the period (in dollars per share)	$ 23.75
Granted (in dollars per share)	$ 44.03
Vested (in dollars per share)	$ 20.00
Forfeited (in dollars per share)	$ 27.12
Balance at the end of the period (in dollars per share)	$ 30.78	$ 23.75
Aggregate Intrinsic Value
Balance at the end of the period (in dollars)	$ 54.1
2012 Plan
Stock-based compensation
Maximum term of options	3 years
Number of shares of common stock available for grant under the plan	2,000,000
Number of shares of common stock available for grant under the plan	1,121,582