Investments in Debt and Equity Securities (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Investments in Debt and Equity Securities
Percent of gross unrealized losses which have been unrealized for greater than one year	55.00%
Gross unrealized losses greater than one year	$ 47,209,000
Gross unrealized losses less than one year	209,318,000
Cost
Equity	249,762,000	$ 246,549,000
Debt	274,110,000	240,878,000
Other	1,520,000	10,807,000
Total	525,392,000	498,234,000
Gains
Equity	111,570,000	91,954,000
Debt	573,000	1,814,000
Other		1,000
Total	112,143,000	93,769,000
Losses
Equity	(4,682,000)	(4,064,000)
Debt	(6,073,000)	(2,262,000)
Total	(10,755,000)	(6,326,000)
Fair Value
Equity	356,650,000	334,439,000
Debt	268,610,000	240,430,000
Other	1,520,000	10,808,000
Total	$ 626,780,000	$ 585,677,000