Derivatives - Schedule of listing currency, fair value and notional amounts of derivative instruments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Derivative [Line Items]
OTC Option Asset	$ 3,500		$ 3,500		$ 3,800
Derivative assets, at fair value	27,337		27,337		21,130
Derivative Assets, Notional Amount	704,492		704,492		836,304
Derivative liabilities, at fair value	22,495		22,495		11,015
Derivative Liabilities, Notional Amount	588,553		588,553		757,406
Realized Gain (Loss) on Derivatives	18,421	$ 2,026	35,872	$ 10,675
Unrealized Gain (Loss) on Derivatives	(4,400)	16,420	(11,658)	(263)
OTC Option Liability	3,300		3,300		2,800
Credit Default Swaps - Protection Purchased
Derivative [Line Items]
Derivative assets, at fair value	16,157		16,157		9,456
Derivative Assets, Notional Amount	118,096		118,096		89,772
Derivative liabilities, at fair value	2,919		2,919		3,230
Derivative Liabilities, Notional Amount	38,582		38,582		49,465
Realized Gain (Loss) on Derivatives	1,232	(1,479)	617	(3,793)
Unrealized Gain (Loss) on Derivatives	3,026	2,843	3,431	(678)
Commodity Contract
Derivative [Line Items]
Derivative assets, at fair value					269
Derivative Assets, Notional Amount					25,168
Derivative liabilities, at fair value					285
Derivative Liabilities, Notional Amount					12,012
Commodity Contract | Short Contracts / Sold
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	(6)	(201)	(6)
Unrealized Gain (Loss) on Derivatives	0	(80)	143	(80)
Credit Default Swaps - Protection Sold
Derivative [Line Items]
Derivative assets, at fair value					205
Derivative Assets, Notional Amount					2,084
Derivative liabilities, at fair value	1,873		1,873		1,319
Derivative Liabilities, Notional Amount	6,019		6,019		5,142
Realized Gain (Loss) on Derivatives	322	1,081	2,017	1,266
Unrealized Gain (Loss) on Derivatives	(556)	(1,181)	(1,916)	(977)
Contracts for Differences | Long Contracts / Purchased
Derivative [Line Items]
Derivative assets, at fair value	1,227		1,227		263
Derivative Assets, Notional Amount	6,072		6,072		3,080
Derivative liabilities, at fair value	9,971		9,971		1,404
Derivative Liabilities, Notional Amount	41,710		41,710		48,152
Realized Gain (Loss) on Derivatives	512	(1,397)	(335)	3,639
Unrealized Gain (Loss) on Derivatives	(12,058)	(5,837)	(7,602)	(12,972)
Contracts for Differences | Short Contracts / Sold
Derivative [Line Items]
Derivative assets, at fair value	5,035		5,035		186
Derivative Assets, Notional Amount	57,496		57,496		6,428
Derivative liabilities, at fair value	1,054		1,054		130
Derivative Liabilities, Notional Amount	47,773		47,773		3,070
Realized Gain (Loss) on Derivatives	14,087	(1,396)	12,150	(3,734)
Unrealized Gain (Loss) on Derivatives	2,505	310	3,925	361
Total Return Swaps | Long Contracts / Purchased
Derivative [Line Items]
Derivative assets, at fair value	1,202		1,202		43
Derivative Assets, Notional Amount	16,366		16,366		1,874
Derivative liabilities, at fair value	4,605		4,605		590
Derivative Liabilities, Notional Amount	20,700		20,700		11,233
Realized Gain (Loss) on Derivatives	3,477	2,488	2,859	12,279
Unrealized Gain (Loss) on Derivatives	3,451	9,990	(2,857)	10,323
Total Return Swaps | Short Contracts / Sold
Derivative [Line Items]
Derivative assets, at fair value	494		494		34
Derivative Assets, Notional Amount	17,853		17,853		9,763
Derivative liabilities, at fair value	429		429
Derivative Liabilities, Notional Amount	14,311		14,311
Realized Gain (Loss) on Derivatives	(108)	(1,112)	(159)	(588)
Unrealized Gain (Loss) on Derivatives	869	795	31	298
Interest Rate Swaps
Derivative [Line Items]
Derivative liabilities, at fair value					220
Derivative Liabilities, Notional Amount					467,956
Realized Gain (Loss) on Derivatives	119	107	119	(350)
Unrealized Gain (Loss) on Derivatives	(530)	(82)	0	267
Interest Rate Swaptions
Derivative [Line Items]
Derivative assets, at fair value	174		174
Derivative Assets, Notional Amount	75,457		75,457
Realized Gain (Loss) on Derivatives	265	(42)	(286)	487
Unrealized Gain (Loss) on Derivatives	(978)	(272)	(419)	(1,848)
Treasury Futures | Short Contracts / Sold
Derivative [Line Items]
Derivative liabilities, at fair value	445		445		280
Derivative Liabilities, Notional Amount	36,418		36,418		10,119
Realized Gain (Loss) on Derivatives	(2,746)	(399)	(2,685)	(1,040)
Unrealized Gain (Loss) on Derivatives	(600)	191	(165)	62
Foreign Currency Forward
Derivative [Line Items]
Derivative assets, at fair value	626		626		4,241
Derivative Assets, Notional Amount	138,207		138,207		228,416
Derivative liabilities, at fair value	367		367
Derivative Liabilities, Notional Amount	77,798		77,798
Realized Gain (Loss) on Derivatives	2,656	5,037	20,071	5,256
Unrealized Gain (Loss) on Derivatives	(1,134)	7,417	(3,984)	4,877
Foreign Currency Options
Derivative [Line Items]
Derivative assets, at fair value	2,422		2,422		6,355
Derivative Assets, Notional Amount	274,945		274,945		283,439
Derivative liabilities, at fair value	307		307		3,527
Derivative Liabilities, Notional Amount	48,814		48,814		144,257
Foreign Currency Options | Long Contracts / Purchased
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	307	256	1,255	(1,484)
Unrealized Gain (Loss) on Derivatives	1,373	1,539	(1,936)	(613)
Foreign Currency Options | Short Contracts / Sold
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	(78)	992	608
Unrealized Gain (Loss) on Derivatives	0	(463)	132	(78)
Catastrophe Risk derivatives
Derivative [Line Items]
Derivative liabilities, at fair value					30
Derivative Liabilities, Notional Amount					6,000
Commodity Future | Long Contracts / Purchased
Derivative [Line Items]
Realized Gain (Loss) on Derivatives			(286)	(271)
Unrealized Gain (Loss) on Derivatives			285	(5)
Commodity Future | Short Contracts / Sold
Derivative [Line Items]
Derivative liabilities, at fair value	68		68
Derivative Liabilities, Notional Amount	118,105		118,105
Realized Gain (Loss) on Derivatives			272	316
Unrealized Gain (Loss) on Derivatives			(269)	(168)
Index Futures | Long Contracts / Purchased
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	(840)	1,144	(840)
Unrealized Gain (Loss) on Derivatives	0	0	0	0
Index Futures | Short Contracts / Sold
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	79	0	(253)
Unrealized Gain (Loss) on Derivatives	0	369	0	441
Bond Futures
Derivative [Line Items]
Derivative liabilities, at fair value	457		457
Derivative Liabilities, Notional Amount	138,323		138,323
Bond Futures | Short Contracts / Sold
Derivative [Line Items]
Derivative assets, at fair value					78
Derivative Assets, Notional Amount					186,280
Realized Gain (Loss) on Derivatives	(1,702)	(273)	(1,702)	(817)
Unrealized Gain (Loss) on Derivatives	232	101	(457)	(253)
Embedded derivatives in reinsurance contracts
Derivative [Line Items]
Derivative liabilities, at fair value	5,715		5,715		2,769
Derivative Liabilities, Notional Amount	20,000		20,000		15,000
Realized Gain (Loss) on Derivatives	0	0	(5)	0
Unrealized Gain (Loss) on Derivatives	366	(21)	211	(127)
Embedded derivatives in deposit contracts
Derivative [Line Items]
Derivative liabilities, at fair value	4,360		4,360		6,520
Derivative Liabilities, Notional Amount	75,000		75,000		75,000
Realized Gain (Loss) on Derivatives	0	0	0	0
Unrealized Gain (Loss) on Derivatives	1,730	(90)	2,160	(310)
Derivative liabilities (embedded)
Derivative [Line Items]
Derivative liabilities, at fair value	10,075		10,075		9,289
Derivative Liabilities, Notional Amount	95,000		95,000		$ 90,000
Realized Gain (Loss) on Derivatives	0	0	(5)	0
Unrealized Gain (Loss) on Derivatives	2,096	(111)	2,371	(437)
Other Contract [Member]
Derivative [Line Items]
Realized Gain (Loss) on Derivatives	0	0	30	0
Unrealized Gain (Loss) on Derivatives	$ 0	$ 780	$ 0	$ 780