Investments - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Aggregate fair value of securities in an unrealized loss position for less than twelve months	$ 81.4	$ 113.9
Aggregate fair value of securities in an unrealized position for more than twelve months	22.3	107.9
Realized gains or losses on available-for-sale securities	$ 0.0	$ 0.0

Amount of realized gain (loss) on 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/recommendedDisclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for less than 12 months.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

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- Definition

Fair value of investment in debt security measured at amortized cost (held-to-maturity), in continuous unrealized loss position for 12 months or longer.

+ References

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- References

No definition available.

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