Investment Securities (Investment gains (losses) for Level 3) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ (37.9)	$ 36.2
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(1.9)	6.9
Level 3 Inputs
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(5.1)	0.2
Level 3 Inputs \| Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ (5.1)	$ 0.2

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7(c))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82895926&loc=d3e24584-111560

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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