Earnings per share (Calculation of Basic and Diluted Earnings Per Share) (Details) - USD ($)	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net income	$ 10,980,715	$ 19,099,901
Less: Earnings allocated to participating Series D convertible preferred stock	(2,624,264)	(4,638,091)
Less: Earnings allocated to participating nonvested shares		(46,703)
Net income for basic and diluted earnings per share	$ 8,356,451	$ 14,415,107
Denominator:
Denominator for basic and diluted earnings per share	50,948,841	49,727,731
Earnings per share:
Basic and diluted	$ 0.16	$ 0.29

Amount of paid and unpaid cash, stock, and paid-in-kind (PIK) dividends declared, for example, but not limited to, common and preferred stock.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities, and addition from assumption of issuance of common shares for dilutive potential common shares; of income (loss) available to common shareholders.

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Amount of undistributed earnings (loss) allocated to participating securities for the basic earnings (loss) per share or per unit calculation under the two-class method.

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Average number of shares or units issued and outstanding that are used in calculating basic and diluted earnings per share (EPS).

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