GOODWILL AND INTANGIBLE ASSETS - Gross amount and accumulated amortization by category (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross amount	$ 531,359	$ 531,359
Accumulated amortization and impairment	(254,077)	(66,534)
Total intangible assets	277,282	464,825
Impairment charges	3,500
Customer lists, trademark and non-compete - Media Pro
Finite-Lived Intangible Assets [Line Items]
Gross amount	9,700	9,700
Accumulated amortization and impairment	(9,598)	(9,244)
Total intangible assets	102	456
Customer relationships and customers lists - Ternian
Finite-Lived Intangible Assets [Line Items]
Gross amount	13,330	13,330
Accumulated amortization and impairment	(4,999)	(3,666)
Total intangible assets	8,331	9,664
Coverholders
Finite-Lived Intangible Assets [Line Items]
Gross amount	63,565	63,565
Accumulated amortization and impairment	(6,622)	(1,324)
Total intangible assets	56,943	62,241
Large brokers
Finite-Lived Intangible Assets [Line Items]
Gross amount	46,641	46,641
Accumulated amortization and impairment	(3,888)	(777)
Total intangible assets	42,753	45,864
SME brokers
Finite-Lived Intangible Assets [Line Items]
Gross amount	14,126	14,126
Accumulated amortization and impairment	(1,471)	(294)
Total intangible assets	12,655	13,832
MGA contract(1)
Finite-Lived Intangible Assets [Line Items]
Gross amount	4,131	4,131
Accumulated amortization and impairment	(4,131)	(148)
Total intangible assets	0	3,983
Impairment charges	3,500
U.S. state licenses
Finite-Lived Intangible Assets [Line Items]
Gross amount	26,036	26,036
Total intangible assets	26,036	26,036
Syndicate capacity
Finite-Lived Intangible Assets [Line Items]
Gross amount	94,748	94,748
Total intangible assets	94,748	94,748
Avaibel | VOBA - Novae
Finite-Lived Intangible Assets [Line Items]
Gross amount	2,140	2,140
Accumulated amortization and impairment	(2,140)	(977)
Total intangible assets	0	1,163
Novae | VOBA - Novae
Finite-Lived Intangible Assets [Line Items]
Gross amount	256,942	256,942
Accumulated amortization and impairment	(221,228)	(50,104)
Total intangible assets	$ 35,714	$ 206,838