Summary of Quarterly Results of Operations (unaudited) - Schedule of Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Net interest income and total other revenue	$ 31,470	$ 28,253	$ 32,346	$ 30,263	$ 24,464	$ 23,497	$ 23,620	$ 20,817
Net income attributable to common stockholders	$ 24,754	$ 20,787	$ 25,320	$ 23,475	$ 19,155	$ 17,439	$ 17,108	$ 16,249	$ 94,336	$ 69,951	$ 59,340
Net income per share of common and Class A common stock:
Basic and diluted	$ 0.41	$ 0.35	$ 0.52	$ 0.48	$ 0.40	$ 0.43	$ 0.42	$ 0.45	$ 1.74	$ 1.69	$ 1.81