Share-Based Compensation (Details 3) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Restricted stock
Summary of changes in restricted stock and related information
Unvested at the beginning of the period (in shares)	608,000
Granted (in shares)	120,000
Vested (in shares)	(182,762)	(200,508)	(191,965)
Forfeited (in shares)	(20,000)
Unvested at the end of the period (in shares)	525,000	608,000
Weighted-Average Grant Date Fair Value, beginning of the period (in dollars per share)	$ 53.03
Weighted-Average Grant Date Fair Value Granted (in dollars per share)	$ 73.65	$ 56.57	$ 47.12
Weighted-Average Grant Date Fair Value Vested (in dollars per share)	$ 44.44
Weighted-Average Grant Date Fair Value Forfeited (in dollars per share)	$ 56.70
Weighted-Average Grant Date Fair Value, end of the period (in dollars per share)	$ 60.60	$ 53.03
Total fair value of awards vested during the period	$ 8.1	$ 9.2	$ 8.9
Unrecognized compensation cost	$ 14.9
Period for recognition of unrecognized compensation cost	2 years 9 months 18 days
Cash-settled restricted stock units
Summary of changes in restricted stock and related information
Unvested at the beginning of the period (in shares)	190,000
Granted (in shares)	15,000
Vested (in shares)	(25,000)
Forfeited (in shares)	(6,000)
Unvested at the end of the period (in shares)	174,000

Unrecognized cost of unvested share-based compensation awards.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 10

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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.

+ References

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

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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).

+ References

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

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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.

+ References

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

-Publisher FASB

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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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-Name Accounting Standards Codification

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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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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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