Fair Value Measurements Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	$ 230,227	$ 283,322
Municipal bonds	1,929	1,851
Total	232,156	285,173
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	230,227	283,322
Municipal bonds	0	0
Total	230,227	283,322
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0	0
Municipal bonds	1,929	1,851
Total	1,929	1,851
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0	0
Municipal bonds	0	0
Total	$ 0	$ 0