General - Investments (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Short-term and long-term investments
Bank overdrafts	$ 3,900		$ 0
Realized gains or losses	0	$ 0
Amortized Cost	393,974		386,169
Gross Unrealized Gains	554		23
Gross Unrealized Losses	(9)		(449)
Estimated Fair Value	394,519		385,743
U.S. Government and agency securities
Short-term and long-term investments
Amortized Cost	110,209		67,870
Gross Unrealized Gains	158		17
Gross Unrealized Losses	(6)		(72)
Estimated Fair Value	110,361		67,815
Obligations of government-sponsored enterprises
Short-term and long-term investments
Amortized Cost	1,503		5,257
Gross Unrealized Losses	(2)		(28)
Estimated Fair Value	1,501		5,229
Corporate debt securities
Short-term and long-term investments
Amortized Cost	257,112		292,392
Gross Unrealized Gains	396		6
Gross Unrealized Losses	(1)		(349)
Estimated Fair Value	257,507		292,049
Certificates of deposit
Short-term and long-term investments
Amortized Cost	25,150		20,650
Estimated Fair Value	$ 25,150		$ 20,650

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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