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Investments in Available-for-Sale Securities (Narrative) (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Abstract]    
Continuous unrealized loss position for less than a year $ 115.2 $ 132.0
Continuous unrealized loss position for over a year $ 32.4 $ 0.0