Unaudited Quarterly Financial Data (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]
Gross written premiums	$ 56,943	$ 55,633	$ 57,753	$ 52,862	$ 47,466	$ 47,823	$ 50,107	$ 43,082	$ 223,191	$ 188,478	$ 177,009
Revenues	50,742	47,839	45,508	42,687	40,353	34,868	33,676	32,718	186,776	141,615	80,596
Net income	5,924	4,201	8,495	6,281	6,871	7,982	6,057	5,257	24,901	26,167	22,273
Comprehensive income	$ 7,422	$ 5,840	$ 10,669	$ 7,354	$ 1,216	$ 7,981	$ 8,947	$ 7,383	$ 31,285	$ 25,527	$ 20,710
Earnings Per Share, Basic [Abstract]
Earnings per share - basic (in dollars per share)	$ 0.28	$ 0.20	$ 0.41	$ 0.30	$ 0.33	$ 0.24	$ 0.00	$ 0.00
Basic earnings per share - Class A common stock					0.00	0.19	0.42	0.37
Basic earnings per share - Class B common stock					0.00	0.21	0.19	0.07
Diluted earnings per share [Abstract]
Earnings per share - diluted (in dollars per share)	$ 0.27	$ 0.20	$ 0.40	$ 0.29	0.32	0.24	0.00	0.00
Diluted earnings per share - Class A common stock					0.00	0.19	0.42	0.37
Diluted earnings per share - Class B common stock					$ 0.00	$ 0.20	$ 0.18	$ 0.07