Fair Value Measurements - Summary of Changes in Fair Value on Available-for-Sale Securities Measured at Fair Value on Recurring Basis using Significant Unobservable Inputs or Level 3 Inputs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value Disclosures [Abstract]
Balance at beginning of period	$ 0	$ 4,549	$ 3,604	$ 4,549
Reclassification of equity securities without a readily determinable fair value to other assets	0	0	(3,604)	0
Balance at end of period	$ 0	$ 4,549	$ 0	$ 4,549