Investments in Debt and Equity Securities (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Investments in Debt and Equity Securities
Gross unrealized losses greater than one year	$ 30,507,000	$ 49,975,000
Percent of gross unrealized losses which have been unrealized for greater than one year	88.00%	59.00%
Gross unrealized losses less than one year	$ 10,533,000	$ 148,638,000
Cost
Equity	255,339,000	251,226,000
Debt	287,991,000	278,030,000
Other	7,848,000	3,075,000
Total	551,178,000	532,331,000
Gains
Equity	115,977,000	64,954,000
Debt	8,400,000	1,718,000
Total	124,377,000	66,672,000
Losses
Equity	(6,337,000)	(9,105,000)
Debt	(888,000)	(4,955,000)
Total	(7,225,000)	(14,060,000)
Fair Value
Equity	364,979,000	307,075,000
Debt	295,503,000	274,793,000
Other	7,848,000	3,075,000
Total	$ 668,330,000	$ 584,943,000