Schedule II - Condensed Financial Information of Registrant, Statements of Cash Flows (Details) - USD ($) $ in Thousands			3 Months Ended								12 Months Ended
	Jun. 02, 2017	Aug. 02, 2016	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
Condensed Financial Statements, Captions [Line Items]
Comprehensive income			$ 7,422	$ 5,840	$ 10,669	$ 7,354	$ 1,216	$ 7,981	$ 8,947	$ 7,383	$ 31,285	$ 25,527	$ 20,710
Operating activities:
Net income			5,924	$ 4,201	$ 8,495	6,281	6,871	$ 7,982	$ 6,057	5,257	24,901	26,167	22,273
Adjustments to reconcile net income to net cash used in operating activities:
Deferred tax expense											1,065	561	(879)
Stock compensation expense											652	492	65
Net Cash Provided by (Used in) Investing Activities [Abstract]
Dividends paid from subsidiary	$ 4,500
Financing activities:
Common stock issued, net of transaction costs											0	72,841	0
Common stock issued, stock options exercised											1,077	0	0
Class A common stock issued		$ 72,800									0	0	90
Dividends paid											(5,039)	(2,097)	0
Proceeds from note payable											0	0	2,000
Repayment of note payable											0	(30,000)	0
Debt issuance costs											0	0	(30)
Net cash (used in) provided by financing activities											(3,971)	40,611	1,931
Net change in cash and cash equivalents											30,995	26,208	586
Cash and cash equivalents at beginning of period						50,752				24,544	50,752	24,544	23,958
Cash and cash equivalents at end of period			81,747				50,752				81,747	50,752	24,544
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Comprehensive income											31,285	25,527	20,710
Operating activities:
Net income											24,901	26,167	22,273
Adjustments to reconcile net income to net cash used in operating activities:
Deferred tax expense											429	(65)	61
Stock compensation expense											652	492	65
Equity in undistributed earnings of subsidiaries											(25,303)	(27,734)	(23,141)
Changes in operating assets and liabilities											(1,583)	520	(286)
Net cash used in operating activities											(904)	(620)	(1,028)
Net Cash Provided by (Used in) Investing Activities [Abstract]
Dividends paid from subsidiary											4,500	0
Contribution to subsidiary											0	(40,000)	(2,000)
Net cash provided by (used in) investing activities											4,500	(40,000)	(2,000)
Financing activities:
Common stock issued, net of transaction costs											0	72,841	0
Common stock issued, stock options exercised											1,077	0	0
Class A common stock issued											0	0	90
Dividends paid											(5,039)	(2,097)	0
Proceeds from note payable											0	0	2,000
Repayment of note payable											0	(30,000)	0
Debt issuance costs											0	0	(30)
Net cash (used in) provided by financing activities											(3,962)	40,744	2,060
Net change in cash and cash equivalents											(366)	124	(968)
Cash and cash equivalents at beginning of period						$ 1,868				$ 1,744	1,868	1,744	2,712
Cash and cash equivalents at end of period			$ 1,502				$ 1,868				$ 1,502	$ 1,868	$ 1,744