Investment Securities - Investment gains (losses) for Level 3 (Details) - Level 3 - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading, unrealized gain (loss)	$ 260.1	$ 54.4	$ 276.0	$ 154.4
Other long-term investments
Debt and Equity Securities, FV-NI [Line Items]
Debt securities, trading, unrealized gain (loss)	$ 260.1	$ 54.4	$ 276.0	$ 154.4

Amount of unrealized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in net income (trading).

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27357-111563

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