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, 2018		Jun. 30, 2017		Jun. 30, 2018		Jun. 30, 2017
Level 3 assets
Balance, beginning of period		$ 265,538		$ 252,432		$ 246,544		$ 251,596
Realized gains (losses)		8,046		5,152		8,539		(361)
Movement in unrealized gains (losses)		(5,406)		10,596		226		17,092
Purchases and issuances		7,343		18,187		38,010		34,862
Sales		0		(25,045)		(13,016)		(35,329)
Settlements		(12,816)		(7,527)		(24,449)		(13,513)
Net transfers into (out of) Level 3		(31,412)		(5,692)		(24,561)		(6,244)
Balance, end of period		231,293		248,103		231,293		248,103
Movement in total gains (losses) above relating to instruments still held at the reporting date		(5,000)		4,280		657		10,411
Derivative contracts - net
Balance, beginning of period		334		163		22		2
Realized gains (losses)		0		0		0		0
Movement in unrealized gains (losses)		(277)		377		35		538
Purchases and issuances		0		0		0		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		57		540		57		540
Movement in total gains (losses) above relating to instruments still held at the reporting date		(277)		377		35		538
U.S. Government and Government Agencies
Level 3 assets
Balance, beginning of period		5,034		19,308		18,290		24,751
Realized gains (losses)		0		(19)		(354)		(38)
Movement in unrealized gains (losses)		(14)		81		100		(274)
Purchases and issuances		0		0		0		0
Sales		0		0		(13,016)		0
Settlements		0		(311)		0		(622)
Net transfers into (out of) Level 3		0	[1]	0	[1]	0	[2]	(4,758)	[2]
Balance, end of period		5,020		19,059		5,020		19,059
Movement in total gains (losses) above relating to instruments still held at the reporting date		(14)		96		(50)		(278)
Corporate
Level 3 assets
Balance, beginning of period		2,128		9,899		1,292		20,085
Realized gains (losses)		0		27		0		27
Movement in unrealized gains (losses)		11		(30)		54		(19)
Purchases and issuances		0		0		0		0
Sales		0		(9,730)		0		(20,014)
Settlements		(92)		0		(193)		(31)
Net transfers into (out of) Level 3		0	[1]	1,018	[1]	894	[2]	1,136	[2]
Balance, end of period		2,047		1,184		2,047		1,184
Movement in total gains (losses) above relating to instruments still held at the reporting date		11		(31)		54		(32)
Derivative contracts - net
Transfers out of Level 3				(100)	[2]			(100)
Transfers Into Level 3				1,100	[2]			1,200
Residential mortgage-backed securities
Level 3 assets
Balance, beginning of period		6,937		3,381		0		0
Realized gains (losses)		0		0		0		17
Movement in unrealized gains (losses)		(10)		0		(10)		(526)
Purchases and issuances		0		0		980		0
Sales		0		0		0		0
Settlements		0		0		0		(173)
Net transfers into (out of) Level 3		(980)	[1]	(3,381)	[1]	4,977	[2]	682	[2]
Balance, end of period		5,947		0		5,947		0
Movement in total gains (losses) above relating to instruments still held at the reporting date		(10)		0		(10)		0
Derivative contracts - net
Transfers out of Level 3						(1,000)		(3,400)	[2]
Transfers Into Level 3						6,000		4,100	[2]
Commercial mortgage-backed securities
Level 3 assets
Balance, beginning of period				101				99
Realized gains (losses)				21				21
Movement in unrealized gains (losses)				(20)				(18)
Purchases and issuances				0				0
Sales				0				0
Settlements				(24)				(24)
Net transfers into (out of) Level 3				0	[1]			0	[2]
Balance, end of period				78				78
Movement in total gains (losses) above relating to instruments still held at the reporting date				(20)				(18)
Other asset-backed securities
Level 3 assets
Balance, beginning of period		19,737		3,950		5,232		1,131
Realized gains (losses)		0		56		0		86
Movement in unrealized gains (losses)		0		1,010		(28)		1,111
Purchases and issuances		0		6,890		15,407		6,890
Sales		0		(1,446)		0		(1,446)
Settlements		0		(240)		(874)		(286)
Net transfers into (out of) Level 3		(19,737)	[1]	(3,329)	[1]	(19,737)	[2]	(595)	[2]
Balance, end of period		0		6,891		0		6,891
Movement in total gains (losses) above relating to instruments still held at the reporting date		0		0		0		0
Derivative contracts - net
Transfers out of Level 3	[2]			(6,700)				(6,700)
Transfers Into Level 3				3,400	[2]			6,100
Short-term investments
Level 3 assets
Balance, beginning of period				11,665				0
Realized gains (losses)				0				0
Movement in unrealized gains (losses)				0				0
Purchases and issuances				0				11,693
Sales				(11,662)				(11,662)
Settlements				0				(28)
Net transfers into (out of) Level 3				0	[1]			0	[2]
Balance, end of period				3				3
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		231,368		203,965		221,708		205,528
Realized gains (losses)		8,046		5,067		8,893		(474)
Movement in unrealized gains (losses)		(5,116)		9,178		75		16,280
Purchases and issuances		7,343		11,297		21,623		16,279
Sales		0		(2,207)		0		(2,207)
Settlements		(12,724)		(6,952)		(23,382)		(12,349)
Net transfers into (out of) Level 3		(10,695)	[1]	0	[1]	(10,695)	[2]	(2,709)	[2]
Balance, end of period		218,222		220,348		218,222		220,348
Movement in total gains (losses) above relating to instruments still held at the reporting date		$ (4,710)		$ 3,858		$ 628		$ 10,201

[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.

[2] Net transfers include both movement in and movement out of Level 3. RMBS for the six months ended June 30, 2018, includes $6.0 million of incoming transfers partially offset by $1.0 million of outgoing transfers. 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.