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Summary of Significant Accounting Policies Earnings per share (Details)
3 Months Ended 12 Months Ended
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Summary of Significant Accounting Policies [Abstract]                      
Weighted-average common shares outstanding - basic 48,335,865 48,332,878 47,944,193 44,367,499 41,418,470 41,292,819 40,994,082 35,182,811 47,251,061 39,743,433 32,306,322
Dilutive effect of equity-based incentive awards                 809,586 1,514,864 2,775,289
Weighted-average common shares outstanding - diluted 48,797,304 48,332,878 48,938,766 45,771,228 41,418,470 42,857,880 42,453,782 36,448,005 48,060,647 41,258,297 35,081,611
Anti-dilutive equity-based incentive awards excluded from the computation of diluted income (loss) per share                 522,552 77,760 14,768