Revenue from Contracts with Customers (Narrative) (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Deferred Revenue, Revenue Recognized	$ 259.2
Test Preparation [Member] \| Long-term Contract with Customer [Member]
Disaggregation of Revenue [Line Items]
Revenue, Remaining Performance Obligation, Amount	$ 5.6
Revenue Remaining Performance Obligation Percentage of Revenue Expected to be Recognized Over Next 12 Months	81.00%
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Percentage of Revenue	80.00%
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Percentage of Revenue	20.00%
U.S. [Member]
Disaggregation of Revenue [Line Items]
Percentage of Revenue	76.00%
Non U.S. [Member]
Disaggregation of Revenue [Line Items]
Percentage of Revenue	24.00%

Percentage of Revenue Recognized During Period from Contract with Customer, Excluding Assessed Tax

No definition available.

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- Definition

Revenue Remaining Performance Obligation Percentage of Revenue Expected to be Recognized Over Next 12 Months

+ References

No definition available.

+ Details

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- Definition

Amount of revenue recognized that was previously reported as deferred or unearned revenue.

+ 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.

+ Details

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- Definition

Amount of transaction price allocated to performance obligation that has not been recognized as revenue.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130556-203045

+ Details

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