Investments - Restricted Cash and Cash Equivalents and Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Restricted Cash and Investments Items [Line Items]
Restricted cash	$ 80,870	$ 59,081	$ 381,698
Restricted investments	1,094,395	1,448,672
Restricted cash and investments	1,175,265	1,507,753
Liability for securities purchased	44,996	0
Third Party Agreements
Restricted Cash and Investments Items [Line Items]
Restricted cash	22,652	21,447
Restricted investments	60,082	65,678
Amortized cost	59,951	65,539
Related Party Agreements
Restricted Cash and Investments Items [Line Items]
Restricted cash	58,218	37,634
Restricted investments	1,066,310	1,382,994
Amortized cost	1,050,781	1,366,873
Related Party Agreements, Liability For Investments Purchased
Restricted Cash and Investments Items [Line Items]
Restricted investments, liability	$ (31,997)	$ 0