Fair Value Measurements - Additional information about assets and liabilities measured at fair value on a recurring basis (Details) - USD ($) $ in Thousands		3 Months Ended				9 Months Ended
		Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
Level 3 assets
Balance, beginning of period		$ 248,103		$ 329,744		$ 251,596		$ 406,902
Realized gains (losses)		2,586		4,724		2,225		12,186
Movement in unrealized gains (losses)		7,708		11,215		24,800		6,812
Purchases and issuances		21,744		134,140		56,606		162,320
Sales		(3,770)		(42,420)		(39,099)		(42,420)
Settlements		(10,131)		(19,821)		(23,644)		(73,833)
Net transfers into (out of) Level 3		(9,513)	[1]	(8,337)		(15,757)	[2]	(62,722)
Balance, end of period		256,727		409,245		256,727		409,245
Movement in total gains (losses) above relating to instruments still held at the reporting date		(1,961)		7,385		8,963		10,442
Derivative contracts - net
Balance, beginning of period		540		(9,836)		2		(9,791)
Realized gains (losses)		0		0		0		0
Movement in unrealized gains (losses)		(394)		9,879		144		9,834
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		146		43		146		43
Movement in total gains (losses) above relating to instruments still held at the reporting date		(394)		55		146		10
U.S. Government and Government Agencies
Level 3 assets
Balance, beginning of period		19,059		44,256		24,751		45,063
Realized gains (losses)		0		(37)		(38)		(112)
Movement in unrealized gains (losses)		50		116		(224)		743
Purchases and issuances		0		0		0		0
Sales		0		0		0		0
Settlements		(313)		(685)		(935)		(2,044)
Net transfers into (out of) Level 3		0	[1]	0	[1]	(4,758)	[2]	0	[2]
Balance, end of period		18,796		43,650		18,796		43,650
Movement in total gains (losses) above relating to instruments still held at the reporting date		50		79		227		630
Corporate
Level 3 assets
Balance, beginning of period		1,184		45,566		20,085		53,873
Realized gains (losses)		0		(3,088)		27		(2,991)
Movement in unrealized gains (losses)		1		94		(18)		26
Purchases and issuances		0		0		0		1,955
Sales		0		(30,420)		(20,014)		(30,420)
Settlements		0		0		(31)		(6)
Net transfers into (out of) Level 3		109	[1]	(1,866)	[1]	1,245	[2]	(12,151)	[2]
Balance, end of period		1,294		10,286		1,294		10,286
Movement in total gains (losses) above relating to instruments still held at the reporting date		(1)		17		5		48
Transfers into (out of) Level 3
Fair value asset - Transfers out of Level 3						(100)
Fair value asset - Transfers into Level 3	[1]					1,400
Residential mortgage-backed securities
Level 3 assets
Balance, beginning of period				15		0		3,077
Realized gains (losses)				0		17		(3)
Movement in unrealized gains (losses)				0		(526)		7
Purchases and issuances				0		0		0
Sales				0		0		0
Settlements				0		(173)		(229)
Net transfers into (out of) Level 3				(15)	[1]	682	[2]	(2,852)	[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
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		78		0		99		0
Realized gains (losses)		0		0		21		0
Movement in unrealized gains (losses)		4		(1)		(14)		(1)
Purchases and issuances		698		0		698		0
Sales		0		0		0		0
Settlements		(2)		0		(26)		0
Net transfers into (out of) Level 3		0	[1]	102	[1]	0	[2]	102	[2]
Balance, end of period		778		101		778		101
Movement in total gains (losses) above relating to instruments still held at the reporting date		4		(1)		4		(1)
Other asset-backed securities
Level 3 assets
Balance, beginning of period		6,891		24,649		1,131		50,265
Realized gains (losses)		15		92		101		704
Movement in unrealized gains (losses)		0		1,189		1,111		4,722
Purchases and issuances		15,582		124,843		22,472		125,915
Sales		(3,770)		0		(5,216)		0
Settlements		0		(4,267)		(286)		(37,220)
Net transfers into (out of) Level 3		(2,467)	[1]	(4,095)	[1]	(3,062)	[2]	(1,975)	[2]
Balance, end of period		16,251		142,411		16,251		142,411
Movement in total gains (losses) above relating to instruments still held at the reporting date		0		333		0		4,479
Transfers into (out of) Level 3
Fair value asset - Transfers out of Level 3		(3,100)				(9,800)	[2]	(5,000)
Fair value asset - Transfers into Level 3		700				6,800	[2]	3,000	[2]
Short-term investments
Level 3 assets
Balance, beginning of period		3				0
Realized gains (losses)		0				0
Movement in unrealized gains (losses)		0				0
Purchases and issuances		0				11,693
Sales		0				(11,662)
Settlements		(3)				(31)
Net transfers into (out of) Level 3		0	[1]			0	[2]
Balance, end of period		0				0
Movement in total gains (losses) above relating to instruments still held at the reporting date		0				0
Other investments
Level 3 assets
Balance, beginning of period		220,348		225,094		205,528		264,415
Realized gains (losses)		2,571		7,757		2,097		14,588
Movement in unrealized gains (losses)		8,047		(62)		24,327		(8,519)
Purchases and issuances		5,464		9,297		21,743		34,450
Sales		0		(12,000)		(2,207)		(12,000)
Settlements		(9,813)		(14,869)		(22,162)		(34,334)
Net transfers into (out of) Level 3		(7,155)	[1]	(2,463)	[1]	(9,864)	[2]	(45,846)	[2]
Balance, end of period		219,462		212,754		219,462		212,754
Movement in total gains (losses) above relating to instruments still held at the reporting date		$ (1,620)		$ 6,902		$ 8,581		5,276
Transfers into (out of) Level 3
Fair value asset - Transfers out of Level 3								(61,300)
Fair value asset - Transfers into Level 3								$ 15,500

[1] Net transfers include both movement in and movement out of Level 3. Other asset-backed securities, for the three months ended September 30, 2017, includes $3.1 million of outgoing transfers partially offset by $0.7 million of incoming transfers.

[2] Net transfers include both movement in and movement out of Level 3. Corporate for the nine months ended September 30, 2017, includes $1.4 million of incoming transfers partially offset by $0.1 million of outgoing transfers. RMBS for the nine months ended September 30, 2017, includes $4.1 million of incoming transfers partially offset by $3.4 million of outgoing transfers. Other asset-backed securities, for the nine months ended September 30, 2017, includes $9.8 million of outgoing transfers partially offset by $6.8 million of incoming transfers. Other investments, for the nine months ended September 30, 2016, includes $61.3 million of outgoing transfers partially offset by $15.5 million of incoming transfers. Other asset-backed securities, for the nine months ended September 30, 2016, includes $5.0 million of outgoing transfers partially offset by $3.0 million of incoming transfers.