SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Property and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property and equipment
Total property and equipment, gross	$ 109,423	$ 98,641
Accumulated depreciation	(22,926)	(17,450)
Land	24,920	24,920
Total property and equipment	$ 111,417	106,111
Home office building
Property and equipment
Useful life	25 years
Total property and equipment, gross	$ 68,123	67,279
Furniture
Property and equipment
Useful life	10 years
Office equipment and machines
Property and equipment
Useful life	5 years
Software and computers
Property and equipment
Useful life	3 years
Data processing equipment
Property and equipment
Total property and equipment, gross	$ 24,102	21,750
Other, principally furniture and equipment
Property and equipment
Total property and equipment, gross	$ 17,198	$ 9,612