Nature of Operations and Summary of Significant Accounting Policies - Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Marketable Securities
Total cash and cash equivalents	$ 181,792	$ 146,766	$ 203,035	$ 132,275
Amortized Cost	503,731	410,014
Gross unrealized holding losses	(228)	(116)
Marketable securities	503,503	409,898
Total Assets Measured at Fair Value - Amortized cost	685,523	556,780
Total cash and cash equivalents and available for sale securities, fair market value	$ 685,295	556,664
Marketable Securities
Maximum period for redemption	1 year
Bonds and bond funds
Marketable Securities
Amortized Cost	$ 344,479	410,001
Gross unrealized holding losses	(228)	(116)
Marketable securities	344,251	409,885
Commercial paper
Marketable Securities
Amortized Cost	159,252	13
Marketable securities	$ 159,252	$ 13

Represents the maximum period for redemption of marketable securities classified as current.

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Amount of investment in cash and cash equivalents and debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

No definition available.

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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-Publisher FASB
-Name Accounting Standards Codification
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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 investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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

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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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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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