Investments - Restricted Cash and Cash Equivalents and Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Restricted Cash and Investments Items [Line Items]
Restricted cash	$ 98,174	$ 59,081	$ 33,171
Restricted investments	1,059,994	1,448,672
Restricted cash and investments	1,158,168	1,507,753
Third Party Agreements
Restricted Cash and Investments Items [Line Items]
Restricted cash	22,669	21,447
Restricted investments	62,059	65,678
Amortized cost	62,013	65,539
Related Party Agreements
Restricted Cash and Investments Items [Line Items]
Restricted cash	75,505	37,634
Restricted investments	997,935	1,382,994
Amortized cost	$ 972,375	$ 1,366,873