Investments - (Losses) Gains Related to Equity Securities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Investments, Debt and Equity Securities [Abstract]
Net gains recognized during the period	$ 12,904	$ 2,382	$ 5,304	$ 23,962
Less: net gains recognized during the period on equity securities sold during the period	18,929	1,716	25,005	6,257
Unrealized (losses) gains recognized during the period on equity securities held	$ (6,025)	$ 666	$ (19,701)	$ 17,705