2. Amortized Cost and Fair Value of Debt Securities (Details) $ in Millions	Jun. 30, 2018 USD ($)
Fair Value Disclosures [Abstract]
Mature in less than one year	$ 156.7
Mature in one to five years	170.9
Mature in more than five years	46.1
Total Amortized Cost	373.7
Mature in less than one year	156.4
Mature in one to five years	169.3
Mature in more than five years	45.0
Estimated Fair Value	$ 370.7

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

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