Fair Value of Financial Instruments - Assets in Level 3 (Details) - Assets and liabilities measured at fair value on recurring basis - Significant Unobservable Inputs (Level 3) - Auction rate securities - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017
Fair value assets reconciliation of changes
Balance at the beginning of the period	$ 5,379	$ 5,196
Gain included in other comprehensive income (loss)	92	275
Balance at the end of the period	$ 5,471	$ 5,471