Investments - Schedule of Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 181,785	$ 247,607
Available-for-sale investments	172,370	$ 244,088
2038
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	267
Available-for-sale investments	281
2039
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,990
Available-for-sale investments	3,068
2042
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,524
Available-for-sale investments	8,766
2043
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	14,396
Available-for-sale investments	13,728
2044
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	23,437
Available-for-sale investments	22,535
2045
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	25,868
Available-for-sale investments	24,468
2046
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	40,006
Available-for-sale investments	37,703
2047
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	57,395
Available-for-sale investments	54,082
2048
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7,902
Available-for-sale investments	$ 7,739

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount of 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://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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