CONSOLIDATED STATEMENTS OF INCOME (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
REVENUES
Premiums earned, affiliated	$ 603,233	$ 586,609	$ 566,103
Other income, affiliated	143	1,021	1,214
LOSSES AND EXPENSES
Losses and settlement expenses, affiliated	417,259	385,708	368,722
Amortization of deferred policy acquisition costs, affiliated	107,854	106,931	101,090
Other underwriting expenses, affiliated	76,932	69,560	68,305
Other expense, affiliated	$ 1,825	$ 1,860	$ 1,822