Investment Securities (Investment gains (losses) for Level 3) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 48.2	$ 76.4	$ 257.8	$ 48.4
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	52.3	20.1	154.1	20.5
Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	54.4	4.0	154.4	2.1
Level 3 Inputs | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 54.4	$ 4.0	$ 154.4	$ 2.1