Investment Securities (Investment gains (losses) for Level 3) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 220.2	$ (37.9)
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	112.7	(1.9)
Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	114.0	(5.1)
Level 3 Inputs | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 114.0	$ (5.1)