Investment Securities (Investment gains (losses) for Level 3) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ (10.6)	$ 9.9	$ 209.6	$ (28.0)
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	(10.9)	2.3	101.8	0.4
Level 3 Inputs
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	(14.0)	3.2	100.0	(1.9)
Level 3 Inputs | Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ (14.0)	$ 3.2	$ 100.0	$ (1.9)