SIGNIFICANT ACCOUNTING POLICES (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	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Gross premiums written									$ (6,910,065)	$ (5,556,273)	$ (4,970,208)
Underwriting income (loss)	$ 194,664	$ (59,026)	$ (115,726)	$ (143,737)	$ (26,130)	$ 512,853	$ (57,012)	$ (16,385)
Revenue									32,000
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred income tax expense (benefit)										13,000
Unrealized gains on equity securities, net of tax, in AOCI					70,000					70,000
Accounting Standards Update 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in tax rate, deferred tax liability, provisional income tax benefit										2,000
Retained earnings | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of ASU					(69,604)					(69,604)	0		$ 0
Retained earnings | Accounting Standards Update 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of adoption of ASU					$ 2,106					2,106	0		$ 0
Accounting Standards Update 2018-11 | Scenario, Forecast
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, liability												$ 149,000
Operating Segments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Gross premiums written									(6,910,065)	(5,556,273)	(4,970,208)
Underwriting income (loss)									(123,827)	413,326	(279,073)
Operating Segments | Insurance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Gross premiums written									(3,797,592)	(2,814,918)	(2,432,475)
Underwriting income (loss)									(77,298)	241,642	(22,630)
Operating Segments | Insurance | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Gross premiums written										313,000	288,000
Underwriting income (loss)										14,000	15,000
Operating Segments | Reinsurance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Gross premiums written									(3,112,473)	(2,741,355)	(2,537,733)
Underwriting income (loss)									$ (46,529)	171,684	(256,443)
Operating Segments | Reinsurance | Restatement Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Gross premiums written										(313,000)	(288,000)
Underwriting income (loss)										$ (14,000)	$ (15,000)