Stockholders' equity (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Stockholders Equity [Line Items]
Balance as of January 1, 2019	$ 748,917,428
Net income	10,980,715	$ 19,099,901
Other comprehensive income	14,670,801
Balance as of March 31, 2019	774,568,944
Preferred Stock [Member] | Series B Preferred Stock [Member]
Stockholders Equity [Line Items]
Balance as of January 1, 2019	$ 100
Balance as of January 1, 2019, shares	1,000,000
Net income
Other comprehensive income
Balance as of March 31, 2019	$ 100
Balance as of March 31, 2019, shares	1,000,000
Common Stock [Member]
Stockholders Equity [Line Items]
Balance as of January 1, 2019	$ 5,097
Balance as of January 1, 2019, shares	50,948,841
Net income
Other comprehensive income
Balance as of March 31, 2019	$ 5,097
Balance as of March 31, 2019, shares	50,948,841
Treasury Stock [Member]
Stockholders Equity [Line Items]
Balance as of January 1, 2019	$ (92,694)
Net income
Other comprehensive income
Balance as of March 31, 2019	(92,694)
Additional Paid-in Capital [Member]
Stockholders Equity [Line Items]
Balance as of January 1, 2019	86,633,582
Net income
Other comprehensive income
Balance as of March 31, 2019	86,633,582
Retained Earnings [Member]
Stockholders Equity [Line Items]
Balance as of January 1, 2019	717,103,890
Net income	10,980,715
Other comprehensive income
Balance as of March 31, 2019	728,084,605
Accumulated Other Comprehensive Income (Loss) [Member]
Stockholders Equity [Line Items]
Balance as of January 1, 2019	(54,732,547)
Net income
Other comprehensive income	14,670,801
Balance as of March 31, 2019	$ (40,061,746)