Marketable Debt Securities Continuous Unrealized Losses (Detail) - Truck, Parts and Other - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Less than 12 Months Fair Value	$ 53.5	$ 252.8
Less than 12 Months Unrealized Losses		0.8
12 Months or Greater Fair value	370.3	397.9
12 Months or Greater Unrealized losses	$ 1.7	$ 3.7

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-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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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer.

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