Marketable Securities (Narrative) (Details) $ in Thousands	3 Months Ended
Jun. 30, 2018 USD ($) security	Mar. 31, 2018 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	$ 2,400	$ 2,433
Amortized cost on available for sale securities held at gross unrealized loss	$ 193,500	$ 198,200
Number of securities in unrealized loss position greater than one year \| security	15	0
Percentage of gross unrealized loss to position (less than)	3.00%
Minimum
Debt Securities, Available-for-sale [Line Items]
Maturity period for highly-rated securities	1 year
Maximum
Debt Securities, Available-for-sale [Line Items]
Maturity period for highly-rated securities	3 years

Available-For-Sale Debt Securities, Maturity Period

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- Definition

Amortized cost of available for sale securities in an unrealized loss position.

+ References

No definition available.

+ Details

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- Definition

Gross Unrealized Loss To Position Value, Percent

+ References

No definition available.

+ Details

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- Definition

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).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer.

+ References

No definition available.

+ Details

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- Definition

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.

+ References

No definition available.

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