Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

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Amount after tax and reclassification adjustments of gain (loss) on foreign currency translation adjustments, foreign currency transactions designated and effective as economic hedges of a net investment in a foreign entity and intra-entity foreign currency transactions that are of a long-term-investment nature.

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Derivative Instruments (Gain (Loss) Recognized in the Consolidated Statements of Operations Related to its Derivative Instruments) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Accumulated other comprehensive loss	$ (3,708)		$ (3,708)
Amount of gain (loss) recognized on derivatives	26,991	$ (3,993)	42,358	$ (2,305)
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	26,147	(3,993)	41,514	(2,305)
Not Designated as Hedging Instrument \| Net realized and unrealized gains (losses) on investments \| Interest rate futures
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	23,077	(115)	29,307	(2,439)
Not Designated as Hedging Instrument \| Net realized and unrealized gains (losses) on investments \| Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	835	241	1,184	135
Not Designated as Hedging Instrument \| Net realized and unrealized gains (losses) on investments \| Total return swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	4,501	0	5,035	0
Not Designated as Hedging Instrument \| Net realized and unrealized gains (losses) on investments \| Credit default swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	1,046	884	5,456	(1,037)
Not Designated as Hedging Instrument \| Net realized and unrealized gains (losses) on investments \| Equity futures
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	7,714	28	9,987	15
Not Designated as Hedging Instrument \| Net foreign exchange gains (losses) \| Foreign currency forward contracts, underwriting and non-investment operations
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(11,616)	(6,374)	(7,174)	369
Not Designated as Hedging Instrument \| Net foreign exchange gains (losses) \| Foreign currency forward contracts, investment operations
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	590	1,343	(2,281)	652
Designated as Hedging Instrument \| Foreign currency forward contracts, investment operations
Derivative Instruments, Gain (Loss) [Line Items]
Accumulated other comprehensive loss	$ 844	$ 0	$ 844	$ 0