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Share-Based Compensation Stock option transactions, exercise price (Details) (USD $)
 6 Months Ended
Jun. 30, 2012
Years
Jun. 30, 2012
Options Outstanding Exercise Price Range 1 [Member]
Years
Jun. 30, 2012
Options Outstanding Exercise Price Range 2 [Member]
Years
Jun. 30, 2012
Options Outstanding Exercise Price Range 3 [Member]
Years
Jun. 30, 2012
Minimum [Member]
Dec. 31, 2011
Minimum [Member]
Jun. 30, 2012
Minimum [Member]
Options Outstanding Exercise Price Range 1 [Member]
Jun. 30, 2012
Minimum [Member]
Options Outstanding Exercise Price Range 2 [Member]
Jun. 30, 2012
Minimum [Member]
Options Outstanding Exercise Price Range 3 [Member]
Jun. 30, 2012
Maximum [Member]
Dec. 31, 2011
Maximum [Member]
Jun. 30, 2012
Maximum [Member]
Options Outstanding Exercise Price Range 1 [Member]
Jun. 30, 2012
Maximum [Member]
Options Outstanding Exercise Price Range 2 [Member]
Jun. 30, 2012
Maximum [Member]
Options Outstanding Exercise Price Range 3 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]                            
Unvested stock options decrease in period (in shares) 88,396                          
Stock-option transactions under the terms of the LTSIP [Roll Forward]                            
Options outstanding, beginning balance (in shares) 1,773,635 743,888 539,174 328,533                    
Exercised (in shares) (162,040)                          
Options outstanding, ending balance (in shares) 1,611,595 743,888 539,174 328,533                    
ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosures [Roll Forward]                            
Weighted-average exercise price, options outstanding, beginning of period (in dollars per share) $ 8.54 $ 4.83 $ 11.78 $ 13.52 $ 4.37 $ 3.70 $ 4.37 $ 11.40 $ 13.10 $ 17.35 $ 17.35 $ 7.84 $ 12.43 $ 17.35
Weighted-average exercise price of options exercised during the period (in dollars per share) $ 4.68       $ 3.70         $ 13.10        
Weighted-average exercise price, options outstanding, end of period (in dollars per share) $ 8.93 $ 4.83 $ 11.78 $ 13.52 $ 4.37 $ 3.70 $ 4.37 $ 11.40 $ 13.10 $ 17.35 $ 17.35 $ 7.84 $ 12.43 $ 17.35
Weighted-average remaining term (in years) 2.0 0.8 2.4 4.1                    
Number of exercisable options at end of period 1,582,197 743,888 539,174 299,135                    
Weighted-average exercise price, options exercisable $ 8.85 $ 4.83 $ 11.78 $ 13.56                    
Number of unvested options at end of period 29,398 0 0 29,398                    
Weighted-average exercise price, unvested options $ 13.10 $ 0.00 $ 0.00 $ 13.10