Financial Instruments and Fair Value Measurements - Additional Information (Details) - USD ($)	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ 107,000		$ 77,000
Realized gains or losses	$ 0	$ 0
Percentage of total fair value available for sale securities in unrealized loss position	87.00%		58.00%
U.S. treasuries \| Level I
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	$ 107,000		$ 77,000
Reported Value Measurement
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Notes payable	443,800,000		462,900,000
Estimate of Fair Value
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Notes payable	$ 446,000,000		$ 462,900,000

Available for sale securities unrealized loss position percentage of total fair value.

No definition available.

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

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- Definition

This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27357-111563

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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

Fair value portion of notes payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19,20,22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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