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Summary of Significant Accounting Policies - Concentration Risk (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) customer item	Dec. 31, 2016 USD ($) customer item
Concentrations of risk
Accrued interest | $	$ 1.7	$ 1.4
Amounts on deposit in excess of federally insured limits approximately	$16.0	$14.0
Payables | Service providers or vendors
Concentrations of risk
Number of service providers	2	3
Concentration risk percentage	40.00%	28.00%
Number of critical suppliers	3
Number of vendors | item	0	0

X

- Definition

Description of credit risk that arises from having significant balances in excess of the Federal Deposit Insurance Corporation or other insurance limits on deposits in financial institutions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13531-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611

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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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- Definition

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6351-108592

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13531-108611

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=84177426&loc=d3e6404-108592

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13537-108611

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- Definition

Represents the amount of accrued interest on marketable securities

+ References

No definition available.

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- Definition

Represents the number of critical suppliers.

+ References

No definition available.

+ Details

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- Definition

Represents the number of service providers.

+ References

No definition available.

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- Definition

Represents the number of vendors.

+ References

No definition available.

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