Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net interest income:
Interest income from securities	$ 8,343	$ 8,293	$ 8,265	$ 8,287	$ 8,275	$ 6,129	$ 4,366	$ 2,419	$ 33,188	$ 21,189	$ 12,267
Interest income from securities, held-to-maturity	2,704	2,956	3,349	3,045	3,165	1,448	0	0	12,054	4,613	0
Interest income from commercial mortgage loans	18,846	15,184	11,968	10,094	9,328	8,025	6,438	4,011	56,092	27,802	16,034
Interest income from subordinate loans	25,623	25,445	21,152	18,610	17,021	19,754	18,238	14,730	90,830	69,743	49,162
Interest expense	(12,275)	(13,187)	(11,917)	(11,482)	(10,740)	(8,786)	(5,258)	(1,757)	(48,861)	(26,541)	(4,356)
Net interest income	43,241	38,691	32,817	28,554	27,049	26,570	23,784	19,403	143,303	96,806	73,107
Operating expenses:
General and administrative expenses (includes equity-based compensation of $4,387 in 2015, $1,576 in 2014 and $3,488 in 2013)	(2,979)	(2,099)	(2,059)	(2,355)	(1,796)	(1,434)	(1,479)	(1,442)	(9,492)	(6,151)	(7,563)
Management fees to related party	(5,294)	(4,097)	(3,887)	(3,341)	(3,236)	(3,193)	(2,966)	(2,565)	(16,619)	(11,960)	(10,012)
Total operating expenses	(8,273)	(6,196)	(5,946)	(5,696)	(5,032)	(4,627)	(4,445)	(4,007)	(26,111)	(18,111)	(17,575)
Income from unconsolidated joint venture	2,972	108	384	0	(69)	(88)	0	0	3,464	(157)	0
Interest income from cash balances	983	239	6	11	9	21	4	0	1,239	34	20
Realized loss on sale of securities	0	0	0	(443)	0	0	0	0	(443)	0	0
Unrealized gain (loss) on securities	(11,618)	(6,926)	(2,273)	3,409	(639)	(2,147)	4,749	2,184	(17,408)	4,147	(3,065)
Foreign currency gain (loss)	(3,121)	(3,998)	6,169	(3,944)	(1,413)	(3,596)	959	0	(4,894)	(4,050)	0
Gain (loss) on derivative instruments (includes unrealized gains (losses) of $(1,063) in 2015, $4,070 in 2014 and $155 in 2013)	3,054	3,929	(6,499)	3,622	2,137	3,026	(1,093)	0	4,106	4,070	(2)
Net income	27,238	25,847	24,658	25,513	22,042	19,159	23,958	17,580	103,256	82,739	52,485
Preferred dividends									(11,884)	(7,440)	(7,440)
Net income available to common stockholders	$ 21,378	$ 23,543	$ 22,798	$ 23,653	$ 20,182	$ 17,299	$ 22,098	$ 15,720	$ 91,372	$ 75,299	$ 45,045
Basic and diluted net income per share of common stock (in dollars per share)	$ 0.32	$ 0.39	$ 0.39	$ 0.47	$ 0.43	$ 0.37	$ 0.51	$ 0.42	$ 1.54	$ 1.72	$ 1.26
Basic weighted average shares of common stock outstanding	67,146,882	59,355,613	58,429,155	49,563,822	46,852,646	46,848,675	42,888,747	37,122,842	58,674,046	43,464,255	35,212,211
Diluted weighted average shares of common stock outstanding	67,754,673	59,934,008	59,022,217	50,171,687	47,085,617	47,068,929	43,099,354	37,341,050	59,273,280	43,684,805	35,679,755