Share-Based Compensation - Activity of stock option plans (Details) (Stock Options, 2012 Plan, USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015
Stock Options \| 2012 Plan
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding, beginning balance (in shares)	1,124,600
Exercised (in shares)	(23,900)
Outstanding, ending balance (in shares)	1,100,700
Exercisable (in shares)	1,100,700
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Outstanding, beginning balance (in dollars per share)	$ 18.84
Exercised (in dollars per share)	$ 20.16
Outstanding, ending balance (in dollars per share)	$ 18.81
Exercisable (in dollars per share)	$ 18.81
Weighted Average Contractual Life [Abstract]
Outstanding weighted average contractual life (in years)	4 years 3 months 12 days
Exercisable weighted average contractual life (in years)	4 years 3 months 0 days
Share Based Compensation Arrangement By Share Based Payment Award Options Vested And Expected To Aggregate Intrinsic Value [Abstract]
Outstanding aggregate intrinsic value	$ 26.6
Exercisable intrinsic value	$ 26.6

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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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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Number of share options (or share units) exercised during the current period.

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