OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications out of AOCI into net income (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
Reclassification out of accumulated comprehensive income into net income available to common shareholders [Line Items]
OTTI losses									$ (9,733)	$ (14,493)	$ (26,210)
Income (loss) before income taxes and interest in income (loss) of equity method investments									12,542	(368,109)	521,802
Foreign exchange gains									29,165	(134,737)	121,295
Income tax (expense) benefit					$ (42,000)				29,486	7,542	(6,340)
Net income (loss) available to common shareholders	$ (198,448)	$ 43,439	$ 92,858	$ 62,546	$ (38,081)	$ (467,740)	$ 85,030	$ 5,014	396	(415,779)	465,462
Non-cash foreign exchange losses									0	24,149	0
Unrealized gains (losses) on available for sale investments | Reclassification out of Accumulated Other Comprehensive Income
Reclassification out of accumulated comprehensive income into net income available to common shareholders [Line Items]
Other realized investment gains (losses)									(95,997)	48,385	(34,213)
OTTI losses									(9,733)	(14,493)	(26,210)
Income (loss) before income taxes and interest in income (loss) of equity method investments									(105,730)	33,892	(60,423)
Income tax (expense) benefit									4,828	(758)	(1,767)
Net income (loss) available to common shareholders									(100,902)	33,134	(62,190)
Cumulative foreign currency translation adjustments, net of tax | Reclassification out of Accumulated Other Comprehensive Income
Reclassification out of accumulated comprehensive income into net income available to common shareholders [Line Items]
Foreign exchange gains									0	(24,149)	0
Income tax (expense) benefit									0	0	0
Net income (loss) available to common shareholders									$ 0	$ (24,149)	$ 0