Fair Value Measurements - Additional information about assets and liabilities measured at fair value on a recurring basis (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2017		Jun. 30, 2016		Jun. 30, 2017		Jun. 30, 2016
Level 3 assets
Balance, beginning of period		$ 252,432		$ 408,729		$ 251,596		$ 426,037
Realized gains (losses)		5,152		552		(361)		7,462
Movement in unrealized gains (losses)		10,596		(4,684)		17,092		(4,403)
Purchases and issuances		18,187		14,232		34,862		28,180
Sales		(25,045)		0		(35,329)		0
Settlements		(7,527)		(10,356)		(13,513)		(54,012)
Net transfers into (out of) Level 3		(5,692)	[1]	(60,585)		(6,244)	[2]	(55,376)
Balance, end of period		248,103		347,888		248,103		347,888
Movement in total gains (losses) above relating to instruments still held at the reporting date		4,280		(4,129)		10,411		3,061
Derivative contracts - net
Balance, beginning of period		163		(9,852)		2		(9,791)
Realized gains (losses)		0		0		0		0
Movement in unrealized gains (losses)		377		16		538		(45)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases, Sales, Issues, Settlements		0		0		0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Inputs Reconciliation, Purchases, Issues								0
Sales		0		0		0		0
Settlements		0		0		0		0
Net transfers into (ouf ot) Level 3		0	[1]	0	[1]	0	[2]	0	[2]
Balance, end of period		540		(9,836)		540		(9,836)
Movement in total gains (losses) above relating to instruments still held at the reporting date		377		16		538		(45)
U.S. Government and Government Agencies
Level 3 assets
Balance, beginning of period		19,308		44,940		24,751		45,063
Realized gains (losses)		(19)		(38)		(38)		(75)
Movement in unrealized gains (losses)		81		34		(274)		627
Purchases and issuances		0		0		0		0
Sales		0		0		0		0
Settlements		(311)		(680)		(622)		(1,359)
Net transfers into (out of) Level 3		0	[1]	0	[1]	(4,758)	[2]	0	[2]
Balance, end of period		19,059		44,256		19,059		44,256
Movement in total gains (losses) above relating to instruments still held at the reporting date		96		(3)		(278)		551
Corporate
Level 3 assets
Balance, beginning of period		9,899		43,764		20,085		53,873
Realized gains (losses)		27		44		27		97
Movement in unrealized gains (losses)		(30)		(28)		(19)		(68)
Purchases and issuances		0		1,786		0		1,955
Sales		(9,730)		0		(20,014)		0
Settlements		0		0		(31)		(6)
Net transfers into (out of) Level 3		1,018	[1]	0	[1]	1,136	[2]	(10,285)	[2]
Balance, end of period		1,184		45,566		1,184		45,566
Movement in total gains (losses) above relating to instruments still held at the reporting date		(31)		17		(32)		31
Transfers into (out of) Level 3
Fair value asset - Transfers out of Level 3		(100)	[1]			(100)
Fair value asset - Transfers into Level 3	[1]	1,100				1,200
Residential mortgage-backed securities
Level 3 assets
Balance, beginning of period		3,381		2,853		0		3,077
Realized gains (losses)		0		0		17		(3)
Movement in unrealized gains (losses)		0		0		(526)		7
Purchases and issuances		0		0		0		0
Sales		0		0		0		0
Settlements		0		(1)		(173)		(229)
Net transfers into (out of) Level 3		(3,381)	[1]	(2,837)	[1]	682	[2]	(2,837)	[2]
Balance, end of period		0		15		0		15
Movement in total gains (losses) above relating to instruments still held at the reporting date		0		0		0		4
Transfers into (out of) Level 3
Fair value asset - Transfers out of Level 3						(3,400)
Fair value asset - Transfers into Level 3						4,100
Commercial mortgage-backed securities
Level 3 assets
Balance, beginning of period		101		0		99		0
Realized gains (losses)		21		0		21		0
Movement in unrealized gains (losses)		(20)		0		(18)		0
Purchases and issuances		0		0		0		0
Sales		0		0		0		0
Settlements		(24)		0		(24)		0
Net transfers into (out of) Level 3		0	[1]	0	[1]	0	[2]	0	[2]
Balance, end of period		78		0		78		0
Movement in total gains (losses) above relating to instruments still held at the reporting date		(20)		0		(18)		0
Other asset-backed securities
Level 3 assets
Balance, beginning of period		3,950		25,929		1,131		50,265
Realized gains (losses)		56		(478)		86		612
Movement in unrealized gains (losses)		1,010		1,034		1,111		3,533
Purchases and issuances		6,890		1,072		6,890		1,072
Sales		(1,446)		0		(1,446)		0
Settlements		(240)		(5,028)		(286)		(32,953)
Net transfers into (out of) Level 3		(3,329)	[1]	2,120	[1]	(595)	[2]	2,120	[2]
Balance, end of period		6,891		24,649		6,891		24,649
Movement in total gains (losses) above relating to instruments still held at the reporting date		0		557		0		4,146
Transfers into (out of) Level 3
Fair value asset - Transfers out of Level 3		(6,700)		(900)		(6,700)	[2]	(900)
Fair value asset - Transfers into Level 3		3,400		3,000		6,100	[2]	3,000	[2]
Non-U.S. Government
Level 3 assets
Balance, beginning of period		0		0		0		0
Realized gains (losses)		0		0		0		0
Movement in unrealized gains (losses)		0		0		0		0
Purchases and issuances		0		0		0		0
Sales		0		0		0		0
Settlements		0		0		0		0
Net transfers into (out of) Level 3		0	[1]	0	[1]	0	[2]	0	[2]
Balance, end of period		0		0		0		0
Movement in total gains (losses) above relating to instruments still held at the reporting date		0		0		0		0
Short-term investments
Level 3 assets
Balance, beginning of period		11,665		0		0		0
Realized gains (losses)		0		0		0		0
Movement in unrealized gains (losses)		0		0		0		0
Purchases and issuances		0		0		11,693		0
Sales		(11,662)		0		(11,662)		0
Settlements		0		0		(28)		0
Net transfers into (out of) Level 3		0	[1]	0	[1]	0	[2]	0	[2]
Balance, end of period		3		0		3		0
Movement in total gains (losses) above relating to instruments still held at the reporting date		0		0		0		0
Other investments
Level 3 assets
Balance, beginning of period		203,965		301,095		205,528		283,550
Realized gains (losses)		5,067		1,024		(474)		6,831
Movement in unrealized gains (losses)		9,178		(5,740)		16,280		(8,457)
Purchases and issuances		11,297		11,374		16,279		25,153
Sales		(2,207)		0		(2,207)		0
Settlements		(6,952)		(4,647)		(12,349)		(19,465)
Net transfers into (out of) Level 3		0	[1]	(59,868)	[1]	(2,709)	[2]	(44,374)	[2]
Balance, end of period		220,348		243,238		220,348		243,238
Movement in total gains (losses) above relating to instruments still held at the reporting date		$ 3,858		$ (4,716)		$ 10,201		(1,626)
Transfers into (out of) Level 3
Fair value asset - Transfers out of Level 3								(59,900)
Fair value asset - Transfers into Level 3								$ 15,500

[1] Net transfers include both movement in and movement out of Level 3. Corporate for the three months ended June 30, 2017, includes $1.1 million of incoming transfers partially offset by $0.1 million of outgoing transfers. Other asset-backed securities, for the three months ended June 30, 2017, includes $6.7 million of outgoing transfers partially offset by $3.4 million of incoming transfers. Other asset-backed securities, for the three months ended June 30, 2016, includes $3.0 million of incoming transfers partially offset by $0.9 million of outgoing transfers.

[2] Net transfers include both movement in and movement out of Level 3. Corporate for the six months ended June 30, 2017, includes $1.2 million of incoming transfers partially offset by $0.1 million of outgoing transfers. RMBS for the six months ended June 30, 2017, includes $4.1 million of incoming transfers partially offset by $3.4 million of outgoing transfers. Other asset-backed securities, for the six months ended June 30, 2017, includes $6.7 million of outgoing transfers partially offset by $6.1 million of incoming transfers. Other investments, for the six months ended June 30, 2016, includes $59.9 million of outgoing transfers partially offset by $15.5 million of incoming transfers. Other asset-backed securities, for the six months ended June 30, 2016, includes $3.0 million of incoming transfers partially offset by $0.9 million of outgoing transfers.