Employee Benefit Plans - Fair Value Assumptions & Compensation Cost (Details) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award
Allocated Share-based Compensation Expense	$ 8,700,000	$ 6,600,000	$ 6,127,000
Options Granted [Abstract]
Risk-free interest rate	0.80%	1.80%	2.80%
Expected life at issuance	5 years 3 months 0 days	5 years 1 month 20 days	6 years 7 months 6 days
Expected volatillity	58.00%	56.70%	58.40%
Expected dividends	$ 0	$ 0	$ 0

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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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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