Share-based Compensation (Details 7) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Performance Stock Units
Share-Based Compensation
Granted (in shares)	206,900	173,107
Closing price on grant date (in dollars per share)	$ 28.16	$ 17.84
Performance period starting price (in dollars per share)	$ 24.94	$ 16.85
Term of award	2 years 11 months 26 days	2 years 11 months 26 days
Volatility (as a percent)	65.06%	69.75%
Risk-free interest rate (as a percent)	0.45%	1.19%
Expected dividend yield (as a percent)	0.00%	0.00%
Fair value per TSR PSU (in dollars per share)	$ 45.37	$ 24.38
Stock trading days	30 days
Total unrecognized compensation cost related to unvested share-based payments	$ 5.90
Weighted-average period of total unrecognized compensation cost related to unvested share-based payments granted	1 year 9 months 14 days
Performance Stock Units \| Minimum
Share-Based Compensation
Percentage of PSUs earned as a percentage of PSUs granted	0.00%
Performance Stock Units \| Maximum
Share-Based Compensation
Percentage of PSUs earned as a percentage of PSUs granted	200.00%
Company specific performance metrics
Share-Based Compensation
Granted (in shares)	186,000	155,000
Fair value per TSR PSU (in dollars per share)	$ 28.16	$ 17.84
TSR Based PSUs
Share-Based Compensation
Granted (in shares)	21,000	17,000
Fair value per TSR PSU (in dollars per share)	$ 45.37	$ 24.38

As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest.

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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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- Definition

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

+ Details

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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).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)(iii)(1)

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph c(1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph b(2)(c)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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- Definition

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

+ References

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 14.D.2)

-URI http://asc.fasb.org/extlink&oid=6793087&loc=d3e301413-122809

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

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

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

-Section D

-Subsection 2

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- Definition

The estimated measure of the 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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- Definition

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

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- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

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- Definition

Represents the number of consecutive common stock trading days.

+ References

No definition available.

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- Definition

Average closing price over the last 30 trading days prior to the performance period start.

+ References

No definition available.

+ Details

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- Definition

Represents the percentage of performance shares earned by a recipient stated as a percentage of performance shares granted.

+ References

No definition available.

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