Fair Value of Financial Instruments - Available-for-sale investments (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Cost
Due in one year or less, Cost	$ 345,420
Due after one year through ten years, Cost	189,524
Due after ten years, Cost	91,570
Total Cost	626,514
Fair Value
Due in one year or less, Fair Value	345,451
Due after one year through ten years, Fair Value	189,507
Due after ten years, Fair Value	91,041
Total Fair Value	625,999
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	215,533	$ 101,480
Fair value of available-for-sale securities, continuous loss position for twelve months or longer	5,471	5,196
Total fair value of available-for-sale securities, continuous loss position	221,004	106,676
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(211)	(288)
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(529)	(804)
Available-for-sale securities, total gross unrealized losses	(740)	(1,092)
Other than temporary impairment losses
Other-than-temporary impairment losses	0
Government debt securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	122,053	79,743
Total fair value of available-for-sale securities, continuous loss position	122,053	79,743
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(109)	(156)
Available-for-sale securities, total gross unrealized losses	(109)	(156)
Corporate debt securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for less than twelve months	93,480	21,737
Total fair value of available-for-sale securities, continuous loss position	93,480	21,737
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for less than 12 months, gross unrealized losses	(102)	(132)
Available-for-sale securities, total gross unrealized losses	(102)	(132)
Auction rate securities
Continuous unrealized loss position, Fair Value
Fair value of available-for-sale securities, continuous loss position for twelve months or longer	5,471	5,196
Total fair value of available-for-sale securities, continuous loss position	5,471	5,196
Continuous unrealized loss position, Gross Unrealized Losses
Available-for-sale securities, continuous loss position for 12 months or greater, gross unrealized losses	(529)	(804)
Available-for-sale securities, total gross unrealized losses	$ (529)	$ (804)