Schedule II - Condensed Financial Information of Registrant, Statements of Income and Comprehensive Income (Loss) (Details) - USD ($) $ 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
Revenues:
Total revenues	$ 50,742	$ 47,839	$ 45,508	$ 42,687	$ 40,353	$ 34,868	$ 33,676	$ 32,718	$ 186,776	$ 141,615	$ 80,596
Expenses [Abstract]
Other expenses									429	2,567	1,992
Interest expense										1,300	1,200
Total expenses									148,255	102,079	47,039
Income before income taxes									38,521	39,536	33,557
Income tax benefit									13,620	13,369	11,284
Net income	5,924	4,201	8,495	6,281	6,871	7,982	6,057	5,257	24,901	26,167	22,273
Other Comprehensive Income (Loss) [Abstract]
Total comprehensive income	$ 7,422	$ 5,840	$ 10,669	$ 7,354	$ 1,216	$ 7,981	$ 8,947	$ 7,383	31,285	25,527	20,710
Parent Company [Member]
Revenues:
Management fees from subsidiaries									2,037	1,815	636
Total revenues									2,037	1,815	636
Expenses [Abstract]
Other operating expenses									2,526	2,395	640
Other expenses									652	492	66
Interest expense									0	1,339	1,230
Total expenses									3,178	4,226	1,936
Loss before equity in net income of subsidiaries									(1,141)	(2,411)	(1,300)
Income tax benefit									(739)	(844)	(432)
Loss before equity in net income of subsidiaries									(402)	(1,567)	(868)
Equity in net income of subsidiaries									25,303	27,734	23,141
Net income									24,901	26,167	22,273
Other Comprehensive Income (Loss) [Abstract]
Equity in other comprehensive earnings (losses) of subsidiaries									6,384	(640)	(1,563)
Total comprehensive income									$ 31,285	$ 25,527	$ 20,710