Investments Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	$ 162,217	$ 139,582
Less than 12 Months, Gross Unrealized Losses	381	332
Twelve Months or Longer, Fair Value	0	0
12 Months or Longer, Gross Unrealized Losses	0	0
Fair Value	162,217	139,582
Gross Unrealized Losses	381	332
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value		17,693
Less than 12 Months, Gross Unrealized Losses		60
Fair Value		17,693
Gross Unrealized Losses		60
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	51,872
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	58
Twelve Months or Longer, Fair Value	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	0
Domestic Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value		18,463
Less than 12 Months, Gross Unrealized Losses		7
Twelve Months or Longer, Fair Value		0
12 Months or Longer, Gross Unrealized Losses		0
Fair Value		18,463
Gross Unrealized Losses		7
Securities and obligations of U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Less than Twelve Months, Fair Value	110,345	103,426
Less than 12 Months, Gross Unrealized Losses	323	265
Twelve Months or Longer, Fair Value	0	0
12 Months or Longer, Gross Unrealized Losses	0	0
Fair Value	110,345	103,426
Gross Unrealized Losses	$ 323	$ 265