Investments - Net Investment Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net investment income included in net income (loss)
Gross investment income	$ 71,199	$ 58,136	$ 38,076
Less: investment income attributable to funds withheld liabilities	(655)	(912)
Less: expenses	(1,647)	(1,253)	(1,880)
Net investment income	68,897	55,971	36,196
Fixed income securities
Net investment income included in net income (loss)
Gross investment income	66,975	55,765	33,467
Commercial levered loans
Net investment income included in net income (loss)
Gross investment income	765	1,114	1,153
Net limited partnerships gains
Net investment income included in net income (loss)
Gross investment income	3,101	1,081	3,240
Other
Net investment income included in net income (loss)
Gross investment income	$ 358	$ 176	$ 216