Investments in Debt and Equity Securities (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Investments in Debt and Equity Securities
Gross unrealized losses greater than one year	$ 52,196,000	$ 49,975,000
Percent of gross unrealized losses which have been unrealized for greater than one year	79.00%	59.00%
Gross unrealized losses less than one year	$ 77,860,000	$ 148,638,000
Cost
Equity	253,840,000	251,226,000
Debt	282,170,000	278,030,000
Other	4,654,000	3,075,000
Total	540,664,000	532,331,000
Gains
Equity	103,643,000	64,954,000
Debt	4,155,000	1,718,000
Total	107,798,000	66,672,000
Losses
Equity	(6,518,000)	(9,105,000)
Debt	(1,715,000)	(4,955,000)
Total	(8,233,000)	(14,060,000)
Fair Value
Equity	350,965,000	307,075,000
Debt	284,610,000	274,793,000
Other	4,654,000	3,075,000
Total	$ 640,229,000	$ 584,943,000