Derivatives - Schedule of listing currency, fair value and notional amounts of derivative instruments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	$ 0	$ 28,035	$ 32,582
Unrealized Gain (Loss) on Derivatives	0	(27,391)	29,396
Credit Default Swaps - Protection Purchased
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	(3,557)	(3,462)
Unrealized Gain (Loss) on Derivatives	0	921	(978)
Credit Default Swaps - Protection Sold
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	(333)	605
Unrealized Gain (Loss) on Derivatives	0	744	(720)
Equity Contracts | Long Contracts / Purchased
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	32,460	58,047
Unrealized Gain (Loss) on Derivatives	0	(15,098)	13,334
Equity Contracts | Short Contracts / Sold
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	4,568	2,608
Unrealized Gain (Loss) on Derivatives	0	(3,608)	4,715
Total Return Swaps | Long Contracts / Purchased
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	16,792	16,863
Unrealized Gain (Loss) on Derivatives	0	(15,864)	16,923
Total Return Swaps | Short Contracts / Sold
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	(17,329)	(15,892)
Unrealized Gain (Loss) on Derivatives	0	1,883	(765)
Interest Rate Swaps
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	0	(3,104)
Unrealized Gain (Loss) on Derivatives	0	0	(1,740)
Interest Rate Swaptions
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	(1,819)	(354)
Unrealized Gain (Loss) on Derivatives	0	1,228	(2,056)
Foreign Currency Forward Contracts
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	(2,849)	(10,470)
Unrealized Gain (Loss) on Derivatives	0	4,403	(3,048)
Foreign Currency Future Options - Purchased | Long Contracts / Purchased
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	(108)	0
Unrealized Gain (Loss) on Derivatives	0	0	0
Foreign Currency Options | Long Contracts / Purchased
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	5,138	(6,716)
Unrealized Gain (Loss) on Derivatives	0	0	1,164
Foreign Currency Options | Short Contracts / Sold
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	(771)	2,183
Unrealized Gain (Loss) on Derivatives	0	0	(80)
Credit Default Swap [Member]
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	3,486	72
Unrealized Gain (Loss) on Derivatives	0	(2,000)	2,000
Embedded derivatives in reinsurance contracts
Derivative [Line Items]
Derivative liabilities, at fair value	31	22
Derivative liabilities, notional amount	20,000	20,000
Realized Gain (Loss) on Derivatives	0	0	0
Unrealized Gain (Loss) on Derivatives	(9)	149	(79)
Derivative liabilities (embedded)
Derivative [Line Items]
Derivative liabilities, at fair value	31	22
Derivative liabilities, notional amount	20,000	20,000
Realized Gain (Loss) on Derivatives	0	0	0
Unrealized Gain (Loss) on Derivatives	(9)	149	(79)
Index Futures | Long Contracts / Purchased
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	403	0
Unrealized Gain (Loss) on Derivatives	0	0	0
Index Futures | Short Contracts / Sold
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	50	0
Unrealized Gain (Loss) on Derivatives	0	0	0
Sovereign Futures [Member] | Long Contracts / Purchased
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	639	0
Unrealized Gain (Loss) on Derivatives	0	0	0
Sovereign Debt Futures | Short Contracts / Sold
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	(1,166)	(7,798)
Unrealized Gain (Loss) on Derivatives	0	0	647
Total Return Swap [Member] | Long Contracts / Purchased
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	(7,569)	0
Unrealized Gain (Loss) on Derivatives	$ 0	$ 0	$ 0