HISTORICAL LOSS AND LAE DEVELOPMENT - Unpaid Losses and Settlement Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Unpaid losses and LAE at beginning of year:
Gross	$ 1,271,503	$ 1,139,337	$ 1,103,785
Ceded	(301,991)	(288,224)	(297,844)
Net	969,512	851,113	805,941
Increase (decrease) in incurred losses and LAE:
Current accident year	478,143	440,452	391,772
Prior accident years	(49,950)	(38,868)	(41,994)
Total incurred	428,193	401,584	349,778
Loss and LAE payments for claims incurred:
Current accident year	(76,050)	(73,392)	(70,540)
Prior accident year	(225,306)	(209,793)	(234,066)
Total paid	(301,356)	(283,185)	(304,606)
Net	1,096,349	969,512	851,113
Unpaid losses and LAE at year end
Gross	1,461,348	1,271,503	1,139,337
Ceded	(364,999)	(301,991)	(288,224)
Net	1,096,349	969,512	851,113
Casualty segment
Increase (decrease) in incurred losses and LAE:
Current accident year	363,015	323,141	292,308
Prior accident years	(33,252)	(17,462)	(32,401)
Property segment
Unpaid losses and LAE at beginning of year:
Ceded	(28,613)
Increase (decrease) in incurred losses and LAE:
Current accident year	94,635	97,161	76,143
Prior accident years	(10,813)	(12,134)	(4,793)
Unpaid losses and LAE at year end
Ceded	(50,495)	(28,613)
Surety segment
Unpaid losses and LAE at beginning of year:
Ceded	(7,079)
Increase (decrease) in incurred losses and LAE:
Current accident year	20,493	20,150	23,321
Prior accident years	(5,885)	(9,272)	$ (4,800)
Unpaid losses and LAE at year end
Ceded	$ (11,834)	$ (7,079)