Investments - Schedule of Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 185,749	$ 247,607
Available-for-sale investments	178,145	$ 244,088
2038
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	269
Available-for-sale investments	284
2039
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,099
Available-for-sale investments	3,203
2042
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	10,099
Available-for-sale investments	9,381
2043
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15,371
Available-for-sale investments	14,823
2044
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	24,183
Available-for-sale investments	23,464
2045
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	28,117
Available-for-sale investments	26,883
2046
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	41,807
Available-for-sale investments	39,789
2047
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	59,890
Available-for-sale investments	57,416
2048
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,914
Available-for-sale investments	$ 2,902

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

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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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