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Financial Instruments (Details 2) - USD ($) $ in Millions | Jun. 30, 2019 | Dec. 31, 2018
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Summary of Contractual Maturities: Available-for-Sale Securities |  |
Due in one year or less, amortized cost | $ 1,227.3 | $ 2,314.6
Due in one year or less, estimated fair value | 1,228.8 | 2,313.4
Due after one year through five years, amortized cost | 1,196.1 | 1,235.9
Due after one year through five years, estimated fair value | 1,202.2 | 1,232.7
Due after five years, amortized cost | 107.0 | 143.5
Due after five years, estimated fair value | 107.1 | 143.2
Amortized cost | 2,530.4 | 3,694.0
Available-for-sale Securities | $ 2,538.1 | $ 3,689.3

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- Definition Amortized cost 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 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454 + Details
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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27198-111563 + Details
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