General - Investments (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Short-term and long-term investments
Other-than-temporary unrealized losses	$ 0		$ 0
Realized gains or losses	0	$ 0
Amortized Cost	379,250		328,364
Gross Unrealized Gains	9		1
Gross Unrealized Losses	(500)		(500)
Estimated Fair Value	378,759		327,865
U.S. Government and agency securities
Short-term and long-term investments
Amortized Cost	57,507		28,313
Gross Unrealized Losses	(123)		(82)
Estimated Fair Value	57,384		28,231
Obligations of government-sponsored enterprises
Short-term and long-term investments
Amortized Cost	8,009		22,139
Gross Unrealized Losses	(44)		(51)
Estimated Fair Value	7,965		22,088
Corporate debt securities
Short-term and long-term investments
Amortized Cost	293,084		270,154
Gross Unrealized Gains	9		1
Gross Unrealized Losses	(333)		(367)
Estimated Fair Value	292,760		269,788
Taxable municipal bonds
Short-term and long-term investments
Amortized Cost			5,000
Estimated Fair Value			5,000
Certificates of deposit
Short-term and long-term investments
Amortized Cost	20,650		2,758
Estimated Fair Value	$ 20,650		$ 2,758