General - Investments (Details) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 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	433,994		328,364
Gross Unrealized Gains	5		1
Gross Unrealized Losses	(751)		(500)
Estimated Fair Value	433,248		327,865
U.S. Government and agency securities
Short-term and long-term investments
Amortized Cost	52,838		28,313
Gross Unrealized Losses	(122)		(82)
Estimated Fair Value	52,716		28,231
Obligations of government-sponsored enterprises
Short-term and long-term investments
Amortized Cost	13,079		22,139
Gross Unrealized Losses	(53)		(51)
Estimated Fair Value	13,026		22,088
Corporate debt securities
Short-term and long-term investments
Amortized Cost	345,965		270,154
Gross Unrealized Gains	5		1
Gross Unrealized Losses	(576)		(367)
Estimated Fair Value	345,394		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	22,112		2,758
Estimated Fair Value	$ 22,112		$ 2,758