BUSINESS COMBINATIONS - Financial results (Details) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Net premiums earned	$ 1,214,469	$ 1,224,075	$ 1,185,548	$ 1,167,402	$ 1,211,495		$ 1,017,131	$ 981,431	$ 938,703	$ 4,791,495	$ 4,148,760	$ 3,705,625
Novae
Business Acquisition [Line Items]
Net premiums earned						$ 140,635
Total revenue						191,929
Total expenses						(197,895)
Net income						$ (5,966)