General - Investments (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Short-term and long-term investments
Other-than-temporary unrealized losses	$ 0		$ 0
Realized gains or losses	0	$ 0
Amortized Cost	336,847		326,589
Gross Unrealized Gains	59		4
Gross Unrealized Losses	(99)		(428)
Estimated Fair Value	336,807		326,165
U.S. Government and agency securities
Short-term and long-term investments
Amortized Cost	5,125		5,524
Gross Unrealized Gains	2
Gross Unrealized Losses			(10)
Estimated Fair Value	5,127		5,514
Obligations of government-sponsored enterprises
Short-term and long-term investments
Amortized Cost	52,538		50,575
Gross Unrealized Gains	12		4
Gross Unrealized Losses	(5)		(54)
Estimated Fair Value	52,545		50,525
Corporate debt securities
Short-term and long-term investments
Amortized Cost	278,034		269,340
Gross Unrealized Gains	45
Gross Unrealized Losses	(94)		(364)
Estimated Fair Value	277,985		268,976
Certificates of deposit
Short-term and long-term investments
Amortized Cost	1,150		1,150
Estimated Fair Value	$ 1,150		$ 1,150