Investment Securities - Investment securities and summary of the gains (losses) related to securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available for sale securities:
Amortized Cost	$ 208,878	$ 266,259
Unrealized gains	20	176
Unrealized losses	(2,564)	(1,237)
Fair value	206,334	265,198
Certificates of deposit
Available for sale securities:
Amortized Cost	8,000	13,000
Unrealized gains	1	1
Unrealized losses	(3)	(2)
Fair value	7,998	12,999
Commercial paper
Available for sale securities:
Amortized Cost		34,836
Unrealized gains		142
Fair value		34,978
Corporate bonds
Available for sale securities:
Amortized Cost	180,861	198,404
Unrealized gains	19	33
Unrealized losses	(2,161)	(995)
Fair value	178,719	197,442
U.S. treasury bills
Available for sale securities:
Amortized Cost	20,017	20,019
Unrealized losses	(400)	(240)
Fair value	$ 19,617	$ 19,779

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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-Publisher FASB
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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
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