GOODWILL AND INTANGIBLE ASSETS - Gross amount and accumulated amortization by category (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Gross amount	$ 120,784	$ 26,036	$ 26,036	$ 26,036
Gross amount		23,030	23,030	29,166
Accumulated amortization	(66,534)	(11,165)	$ (9,356)	$ (13,390)
Total	137,203
Gross amount	531,359	49,066
Total	464,825	37,901
Customer lists, trademark and non-compete - Media Pro
Finite-Lived Intangible Assets [Line Items]
Gross amount	9,700	9,700
Accumulated amortization	(9,244)	(8,832)
Total	456	868
Customer relationships and customers lists - Ternian
Finite-Lived Intangible Assets [Line Items]
Gross amount	13,330	13,330
Accumulated amortization	(3,666)	(2,333)
Total	9,664	10,997
Other intangibles - Aviabel
Finite-Lived Intangible Assets [Line Items]
Gross amount	2,140
Accumulated amortization	(977)
Total	1,163
VOBA - Novae
Finite-Lived Intangible Assets [Line Items]
Gross amount	256,942
Accumulated amortization	(50,104)
Total	206,838
Coverholders
Finite-Lived Intangible Assets [Line Items]
Gross amount	63,565
Accumulated amortization	(1,324)
Total	62,241
Large brokers
Finite-Lived Intangible Assets [Line Items]
Gross amount	46,641
Accumulated amortization	(777)
Total	45,864
SME brokers
Finite-Lived Intangible Assets [Line Items]
Gross amount	14,126
Accumulated amortization	(294)
Total	13,832
MGA contract
Finite-Lived Intangible Assets [Line Items]
Gross amount	4,131
Accumulated amortization	(148)
Total	3,983
U.S. state licenses
Finite-Lived Intangible Assets [Line Items]
Gross amount	26,036	$ 26,036
Syndicate capacity
Finite-Lived Intangible Assets [Line Items]
Gross amount	$ 94,748