Investments - Restricted Cash and Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Restricted Cash and Investments Items [Line Items]
Restricted cash	$ 87,761	$ 123,584	$ 109,697
Restricted investments	4,680,590	4,744,940
Restricted cash and investments	4,768,351	4,868,524
Amortized cost	5,105,382	5,125,794
Third party agreements [Member]
Restricted Cash and Investments Items [Line Items]
Restricted cash	51,116	50,411
Restricted investments	1,209,230	1,320,800
Amortized cost	1,231,954	1,316,305
Related party agreements [Member]
Restricted Cash and Investments Items [Line Items]
Restricted cash	36,458	73,017
Restricted investments	2,397,262	2,294,367
Amortized cost	2,419,659	2,281,672
HTM related party agreements [Member]
Restricted Cash and Investments Items [Line Items]
Restricted investments	1,069,980	1,125,626
Amortized cost	1,071,361	1,097,801
US state regulatory authorities [Member]
Restricted Cash and Investments Items [Line Items]
Restricted cash	187	156
Restricted investments	4,118	4,147
Amortized cost	$ 4,080	$ 4,076