Fair Value Measurements - Additional information about assets and liabilities measured at fair value on a recurring basis (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Level 3 assets
Balance, beginning of period		$ 251,596	$ 426,037
Realized gains (losses)		13,813	16,035
Movement in unrealized gains (losses)		24,706	4,642
Purchases and issuances		67,693	51,102
Sales		(39,099)	(77,197)
Settlements		(45,359)	(83,224)
Net transfers into (out of) Level 3 (1)	[1]	(26,806)	(85,799)
Balance, end of period		246,544	251,596
Movement in total gains (losses) above relating to instruments still held at the reporting date		(132)	7,871
Level 3 liabilities
Balance, beginning of period		2	(9,791)
Realized gains (losses)		0	0
Movement in unrealized gains (losses)		20	9,793
Purchases and Issuances		0	0
Sales		0	0
Settlements		0	0
Net transfers into (out of) Level 3 (1)	[1]	0	0
Balance, end of period		22	2
Movement in total gains (losses) relating to investments still held		22	10
U.S. Government and Government - Related/Supported
Level 3 assets
Balance, beginning of period		24,751	45,063
Realized gains (losses)		(80)	264
Movement in unrealized gains (losses)		(372)	687
Purchases and issuances		0	0
Sales		0	(19,101)
Settlements		(1,251)	(2,162)
Net transfers into (out of) Level 3 (1)	[1]	(4,758)	0
Balance, end of period		18,290	24,751
Movement in total gains (losses) above relating to instruments still held at the reporting date		37	886
Corporate
Level 3 assets
Balance, beginning of period		20,085	53,873
Realized gains (losses)		(50)	(2,991)
Movement in unrealized gains (losses)		57	23
Purchases and issuances		0	2,032
Sales		(20,014)	(30,420)
Settlements		(31)	(6)
Net transfers into (out of) Level 3 (1)	[1]	1,245	(2,426)
Balance, end of period		1,292	20,085
Movement in total gains (losses) above relating to instruments still held at the reporting date		(2)	45
RMBS
Level 3 assets
Balance, beginning of period		0	3,077
Realized gains (losses)		17	(3)
Movement in unrealized gains (losses)		(526)	7
Purchases and issuances		0	0
Sales		0	0
Settlements		(173)	(229)
Net transfers into (out of) Level 3 (1)	[1]	682	(2,852)
Balance, end of period		0	0
Movement in total gains (losses) above relating to instruments still held at the reporting date		0	0
CMBS
Level 3 assets
Balance, beginning of period		99	0
Realized gains (losses)		61	0
Movement in unrealized gains (losses)		(55)	(3)
Purchases and issuances		698	0
Sales		0	0
Settlements		(105)	0
Net transfers into (out of) Level 3 (1)	[1]	(698)	102
Balance, end of period		0	99
Movement in total gains (losses) above relating to instruments still held at the reporting date		0	(3)
Other asset-backed securities
Level 3 assets
Balance, beginning of period		1,131	50,265
Realized gains (losses)		101	668
Movement in unrealized gains (losses)		1,114	4,757
Purchases and issuances		22,472	1,072
Sales		(5,216)	0
Settlements		(957)	(37,220)
Net transfers into (out of) Level 3 (1)	[1]	(13,413)	(18,411)
Balance, end of period		5,232	1,131
Movement in total gains (losses) above relating to instruments still held at the reporting date		0	4,479
Short-term Investments [Member]
Level 3 assets
Balance, beginning of period		0
Realized gains (losses)		0
Movement in unrealized gains (losses)		0
Purchases and issuances		11,693
Sales		(11,662)
Settlements		(31)
Net transfers into (out of) Level 3 (1)	[1]	0
Balance, end of period		0	0
Movement in total gains (losses) above relating to instruments still held at the reporting date		0
Other investments
Level 3 assets
Balance, beginning of period		205,528	283,550
Realized gains (losses)		13,764	18,097
Movement in unrealized gains (losses)		24,468	(10,622)
Purchases and issuances		32,830	47,998
Sales		(2,207)	(27,676)
Settlements		(42,811)	(43,607)
Net transfers into (out of) Level 3 (1)	[1]	(9,864)	(62,212)
Balance, end of period		221,708	205,528
Movement in total gains (losses) above relating to instruments still held at the reporting date		$ (189)	$ 2,454

[1] Net transfers include both movement in and movement out of Level 3. Corporate for 2017 includes incoming transfers of $1.4 million partially offset by $0.1 million of outgoing transfers. RMBS for 2017 includes incoming transfers $4.1 million partially offset by $3.4 million of outgoing transfers. Other asset-backed securities for 2017 includes outgoing transfers of $25.4 million partially offset by $12.0 million of incoming transfers. Corporate for 2016 includes outgoing transfers of $12.2 million partially offset by $9.7 million of incoming transfers. Other asset-backed securities for 2016 includes outgoing transfers $21.4 million partially offset by $3.0 million of incoming transfers. Other investments for 2016 includes $77.7 million of outgoing transfers partially offset by 15.5 million of incoming transfers.