Fair Value (Available-For-Sale Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Fair Value Disclosure [Line Items]
Amortized Cost	$ 141.7	$ 141.3
Gross Unrealized Gains	4.3	0.5
Gross Unrealized Losses	(0.3)	(3.5)
Fair Value	145.7	138.3
U.S. Treasury debt securities
Fair Value Disclosure [Line Items]
Amortized Cost	31.1	23.6
Gross Unrealized Gains	0.2	0.1
Gross Unrealized Losses	(0.1)	(0.1)
Fair Value	31.2	23.6
Corporate/Other debt securities
Fair Value Disclosure [Line Items]
Amortized Cost	110.6	117.7
Gross Unrealized Gains	4.1	0.4
Gross Unrealized Losses	(0.2)	(3.4)
Fair Value	$ 114.5	$ 114.7

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on 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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