Investment Securities (Investment gains (losses) for Level 3) (Details) - Level 3 Inputs - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 25.3	$ (14.0)	$ 15.9	$ 100.0
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Unrealized gains (losses) on investments	$ 25.3	$ (14.0)	$ 15.9	$ 100.0

Amount of unrealized holding gain (loss) recognized in the income statement for investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

No definition available.

Name:	us-gaap_TradingSecuritiesUnrealizedHoldingGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration