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		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Accumulated other comprehensive income (loss)	$ 8,249		$ 4,541
Amount of gain (loss) recognized on derivatives	9,397	$ 617	51,755	$ (1,688)
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	6,133	617	47,647	(1,688)
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	7,199	2,731	35,136	292
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	887	(6)	2,071	129
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	65	(191)	5,521	(1,228)
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	1,960	0	6,995	0
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	1,023	29	12,380	44
Not Designated as Hedging Instrument \| Net foreign exchange losses \| Foreign currency forward contracts, underwriting and non-investment operations
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	(5,912)	(2,596)	(13,086)	(2,227)
Not Designated as Hedging Instrument \| Net foreign exchange losses \| Foreign currency forward contracts, investment operations
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized on derivatives	911	650	(1,370)	1,302
Designated as Hedging Instrument \| Foreign currency forward contracts, investment operations
Derivative Instruments, Gain (Loss) [Line Items]
Accumulated other comprehensive income (loss)	$ 3,264	$ 0	$ 4,108	$ 0