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.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Marketable Securities - Schedule of Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 60,333	$ 51,281	$ 58,750	$ 101,131
Available-for-sale Securities [Abstract]
Unrealized Gains	0	1
Unrealized Losses	(69)	(12)
Cash and cash equivalents and available-for-sale securities, amortized cost	65,885	54,942
Cash and cash equivalents and available-for-sale securities	65,816	54,931
U.S. treasury debt securities
Available-for-sale Securities [Abstract]
Amortized Cost	3,083	3,503
Unrealized Gains	0	0
Unrealized Losses	(8)	(5)
Aggregate Fair Value	3,075	3,498
U.S. corporate debt securities
Available-for-sale Securities [Abstract]
Amortized Cost	13,350	7,943
Unrealized Gains	0	1
Unrealized Losses	(61)	(7)
Aggregate Fair Value	13,289	7,937
Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	49,452	43,496
Cash and cash equivalents, aggregate fair value	49,452	43,496
Cash and Cash Equivalents \| Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	49,452	43,496
Cash and cash equivalents, aggregate fair value	49,452	43,496
Marketable Securities \| U.S. corporate debt securities
Available-for-sale Securities [Abstract]
Amortized Cost	16,433	11,446
Unrealized Gains	0	1
Unrealized Losses	(69)	(12)
Aggregate Fair Value	$ 16,364	$ 11,435