Investments - Investments in Debt and Equity Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Schedule of Investments [Abstract]
Gross unrealized losses greater than one year	$ 22,352	$ 49,975
Percent of gross unrealized losses which have been unrealized for greater than one year	86.00%	59.00%
Gross unrealized losses less than one year	$ 69,567	$ 148,638
Cost
Equity	258,870	251,226
Debt	354,535	278,030
Other	6,356	3,075
Total	619,761	532,331
Gross Unrealized Gains
Equity	144,832	64,954
Debt	8,474	1,718
Other	0	0
Total	153,306	66,672
Gross Unrealized Losses
Equity	(5,990)	(9,105)
Debt	(874)	(4,955)
Other	0	0
Total	(6,864)	(14,060)
Fair Value
Equity	397,712	307,075
Debt	362,135	274,793
Other	6,356	3,075
Total	$ 766,203	$ 584,943