Investment Securities - Investment securities and summary of the gains (losses) related to securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available for sale securities:
Amortized Cost	$ 264,259	$ 251,810
Unrealized gains	1,669	323
Unrealized losses	(540)	(1,369)
Fair value	265,388	250,764
Certificates of deposit
Available for sale securities:
Amortized Cost		5,000
Unrealized gains		1
Fair value		5,001
Commercial paper
Available for sale securities:
Amortized Cost	8,304	7,902
Unrealized gains	1	68
Fair value	8,305	7,970
Corporate bonds
Available for sale securities:
Amortized Cost	255,955	219,236
Unrealized gains	1,668	254
Unrealized losses	(540)	(1,369)
Fair value	$ 257,083	218,121
U.S. treasury bills
Available for sale securities:
Amortized Cost		19,672
Fair value		$ 19,672

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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