Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Earnings Per Share (Details)

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Summary of Significant Accounting Policies - Earnings Per Share (Details) (USD $)
 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, 2010
Sep. 30, 2010
Jun. 30, 2010
Mar. 31, 2010
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Accounting Policies [Abstract]                              
Net income $ 5,094 $ 6,712 $ 4,819 $ 1,735 $ 8,443 $ 11,023 $ 8,390 $ 5,557 $ 8,883 $ 11,300 $ 6,554 $ 778 $ 18,360,133 $ 33,412,933 $ 27,513,666
Basic weighted-average shares outstanding 45,116,164 44,788,160 44,541,025 44,145,093 43,514,982 43,116,861 42,367,242 41,322,276 41,086,998 34,434,352 13,742,472 13,282,284 44,649,275 42,587,818 25,728,314
Effect of dilutive securities:                              
Preferred stock (in shares)                         0 0 10,606,192
Stock options (in shares)                         1,346,188 2,119,314 4,994,730
Warrants (in shares)                         0 0 184,246
Diluted weighted-average shares outstanding 46,508,767 46,247,239 46,359,323 46,128,417 45,554,558 46,308,032 45,907,748 45,823,090 46,147,728 43,445,391 38,412,419 36,587,847 45,995,463 44,707,132 41,513,482
Stock options excluded from calculation of dilutive shares as the effect would be anti-dilutive                         681,688 88,688 856,000