Consolidated Balance Sheets (Parenthetical) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Available-for-sale securities, pledged securities	$ 1,295,225,428	$ 876,121,505
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	50,000,000	50,000,000
Preferred stock, dividend rate (percentage)	8.75%	8.75%
Preferred stock, liquidation preference	$ 25	$ 25
Preferred stock, shares issued (in shares)	1,610,000	1,610,000
Preferred stock, shares outstanding (in shares)	1,610,000	1,610,000
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	450,000,000	450,000,000
Common stock, shares issued (in shares)	22,143,758	17,539,258
Common stock, shares outstanding (in shares)	22,143,758	17,539,258
Assets of consolidated VIEs	$ 1,255,404,335	$ 1,369,120,941
Liabilities of consolidated VIEs	$ 1,228,295,517	$ 1,344,404,080