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The total amount of dividends declared in the period for each class of stock and the contractual amount of dividends (or interest on participating income bonds) that must be paid for the period (for example, unpaid cumulative dividends). Dividends declared in the current period do not include dividends declared in respect of prior-period unpaid cumulative dividends. Preferred dividends that are cumulative only if earned are deducted only to the extent that they are earned.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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Amount of undistributed earnings (loss) allocated to participating securities for the basic earnings (loss) per share or per unit calculation under the two-class method.

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The sum of dilutive potential common shares or units used in the calculation of the diluted per-share or per-unit computation.

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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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Stockholders' Equity - Basic and Diluted Shares Reconciliation (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator (basic and diluted):
Net income	$ 18.5us-gaap_NetIncomeLoss	$ 23.0us-gaap_NetIncomeLoss	$ 25.0us-gaap_NetIncomeLoss	$ 23.2us-gaap_NetIncomeLoss	$ (9.7)us-gaap_NetIncomeLoss	$ 29.1us-gaap_NetIncomeLoss	$ 26.6us-gaap_NetIncomeLoss	$ 30.1us-gaap_NetIncomeLoss	$ 89.7us-gaap_NetIncomeLoss	$ 76.1us-gaap_NetIncomeLoss	$ 79.8us-gaap_NetIncomeLoss
Less: Dividends paid to participating securities									(0.3)us-gaap_DistributedEarnings	(0.2)us-gaap_DistributedEarnings	(0.2)us-gaap_DistributedEarnings
Less: Undistributed earnings available to participating securities									(0.5)	(0.4)	(0.9)
Undistributed and distributed earnings available to common stockholders									$ 88.9	$ 75.5	$ 78.7
Denominator:
Average number of common shares outstanding (shares)									30,238,000	31,056,700	30,986,200
Effect of dilutive stock-based compensation (shares)									118,500	181,600	355,700
Average number of common and potential common shares outstanding (shares)									30,356,500	31,238,300	31,341,900