Financial Information for Guarantor Subsidiaries and Non-Guarantor Subsidiaries - Supplemental Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Condensed Financial Statements
Net cash (used in) provided by operating activities	$ 132.4	$ 279.5
Cash flows from investing activities:
Capital expenditures	(200.5)	(140.2)
Acquisitions of businesses, net of cash acquired	(274.1)	(52.1)
Distributions of capital from equity investments	23.2	22.4
Changes in other assets and liabilities and other	0.0	10.0
Additions to equity method investments	(75.2)	0.0
Other, principally change in intercompany investing activities	0.0	0.0
Net cash used in investing activities	(526.6)	(159.9)
Cash flows from financing activities:
Net payments of long-term debt including proceeds and repurchases of senior notes and term loans	92.6	12.6
Repayment of assumed NYX debt	(288.2)	0.0
Payments of debt issuance and deferred financing costs	(38.5)	(27.7)
Payments on license obligations	(14.0)	(19.5)
Net redemptions of common stock under stock-based compensation plans and other	(21.5)	(3.9)
Other, principally change in intercompany financing activities	0.0	0.0
Net cash provided by (used in) financing activities	(269.6)	(38.5)
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(2.9)	2.8
Increase (decrease) in cash, cash equivalents and restricted cash	(666.7)	83.9
Cash, cash equivalents and restricted cash, beginning of period	834.1	156.9
Cash, cash equivalents and restricted cash, end of period	167.4	240.8
Reportable Legal Entities | SGI (Issuer)
Condensed Financial Statements
Net cash (used in) provided by operating activities	(59.1)	(125.1)
Cash flows from investing activities:
Capital expenditures	(46.1)	(8.9)
Acquisitions of businesses, net of cash acquired	0.0	0.0
Distributions of capital from equity investments	0.0	0.0
Changes in other assets and liabilities and other		0.0
Additions to equity method investments	(0.9)
Other, principally change in intercompany investing activities	48.0	(102.9)
Net cash used in investing activities	1.0	(111.8)
Cash flows from financing activities:
Net payments of long-term debt including proceeds and repurchases of senior notes and term loans	96.6	265.7
Repayment of assumed NYX debt	0.0
Payments of debt issuance and deferred financing costs	(38.5)	(27.7)
Payments on license obligations	0.0	0.0
Net redemptions of common stock under stock-based compensation plans and other	0.0	0.0
Other, principally change in intercompany financing activities	0.0	0.0
Net cash provided by (used in) financing activities	58.1	238.0
Effect of exchange rate changes on cash, cash equivalents and restricted cash	0.0	0.0
Increase (decrease) in cash, cash equivalents and restricted cash	0.0	1.1
Cash, cash equivalents and restricted cash, beginning of period	0.6	1.7
Cash, cash equivalents and restricted cash, end of period	0.6	2.8
Reportable Legal Entities | SGC (Parent)
Condensed Financial Statements
Net cash (used in) provided by operating activities	(50.5)	(101.1)
Cash flows from investing activities:
Capital expenditures	(22.1)	(25.1)
Acquisitions of businesses, net of cash acquired	0.0	0.0
Distributions of capital from equity investments	0.0	0.0
Changes in other assets and liabilities and other		0.0
Additions to equity method investments	0.0
Other, principally change in intercompany investing activities	0.0	0.0
Net cash used in investing activities	(22.1)	(25.1)
Cash flows from financing activities:
Net payments of long-term debt including proceeds and repurchases of senior notes and term loans	0.0	(250.0)
Repayment of assumed NYX debt	0.0
Payments of debt issuance and deferred financing costs	0.0	0.0
Payments on license obligations	(13.8)	(17.0)
Net redemptions of common stock under stock-based compensation plans and other	(19.7)	(3.9)
Other, principally change in intercompany financing activities	(581.6)	507.1
Net cash provided by (used in) financing activities	(615.1)	236.2
Effect of exchange rate changes on cash, cash equivalents and restricted cash	0.0	0.0
Increase (decrease) in cash, cash equivalents and restricted cash	(687.7)	110.0
Cash, cash equivalents and restricted cash, beginning of period	732.6	32.7
Cash, cash equivalents and restricted cash, end of period	44.9	142.7
Reportable Legal Entities | Guarantor Subsidiaries
Condensed Financial Statements
Net cash (used in) provided by operating activities	84.7	327.9
Cash flows from investing activities:
Capital expenditures	(89.3)	(67.9)
Acquisitions of businesses, net of cash acquired	(9.6)	(26.3)
Distributions of capital from equity investments	0.0	0.0
Changes in other assets and liabilities and other		7.5
Additions to equity method investments	0.0
Other, principally change in intercompany investing activities	0.0	0.0
Net cash used in investing activities	(98.9)	(86.7)
Cash flows from financing activities:
Net payments of long-term debt including proceeds and repurchases of senior notes and term loans	0.0	0.0
Repayment of assumed NYX debt	0.0
Payments of debt issuance and deferred financing costs	0.0	0.0
Payments on license obligations	(0.2)	(2.5)
Net redemptions of common stock under stock-based compensation plans and other	(1.8)	0.0
Other, principally change in intercompany financing activities	15.1	(237.9)
Net cash provided by (used in) financing activities	13.1	(240.4)
Effect of exchange rate changes on cash, cash equivalents and restricted cash	0.0	0.0
Increase (decrease) in cash, cash equivalents and restricted cash	(1.1)	0.8
Cash, cash equivalents and restricted cash, beginning of period	43.9	41.0
Cash, cash equivalents and restricted cash, end of period	42.8	41.8
Reportable Legal Entities | Non-Guarantor Subsidiaries
Condensed Financial Statements
Net cash (used in) provided by operating activities	163.7	177.4
Cash flows from investing activities:
Capital expenditures	(43.0)	(38.3)
Acquisitions of businesses, net of cash acquired	(264.5)	(25.8)
Distributions of capital from equity investments	23.2	22.4
Changes in other assets and liabilities and other		2.5
Additions to equity method investments	(74.3)
Other, principally change in intercompany investing activities	0.0	(166.3)
Net cash used in investing activities	(358.6)	(205.5)
Cash flows from financing activities:
Net payments of long-term debt including proceeds and repurchases of senior notes and term loans	(4.0)	(3.1)
Repayment of assumed NYX debt	(288.2)
Payments of debt issuance and deferred financing costs	0.0	0.0
Payments on license obligations	0.0	0.0
Net redemptions of common stock under stock-based compensation plans and other	0.0	0.0
Other, principally change in intercompany financing activities	518.5	0.0
Net cash provided by (used in) financing activities	226.3	(3.1)
Effect of exchange rate changes on cash, cash equivalents and restricted cash	(2.9)	2.8
Increase (decrease) in cash, cash equivalents and restricted cash	28.5	(28.4)
Cash, cash equivalents and restricted cash, beginning of period	60.2	82.6
Cash, cash equivalents and restricted cash, end of period	88.7	54.2
Eliminating Entries
Condensed Financial Statements
Net cash (used in) provided by operating activities	(6.4)	0.4
Cash flows from investing activities:
Capital expenditures	0.0	0.0
Acquisitions of businesses, net of cash acquired	0.0	0.0
Distributions of capital from equity investments	0.0	0.0
Changes in other assets and liabilities and other		0.0
Additions to equity method investments	0.0
Other, principally change in intercompany investing activities	(48.0)	269.2
Net cash used in investing activities	(48.0)	269.2
Cash flows from financing activities:
Net payments of long-term debt including proceeds and repurchases of senior notes and term loans	0.0	0.0
Repayment of assumed NYX debt	0.0
Payments of debt issuance and deferred financing costs	0.0	0.0
Payments on license obligations	0.0	0.0
Net redemptions of common stock under stock-based compensation plans and other	0.0	0.0
Other, principally change in intercompany financing activities	48.0	(269.2)
Net cash provided by (used in) financing activities	48.0	(269.2)
Effect of exchange rate changes on cash, cash equivalents and restricted cash	0.0	0.0
Increase (decrease) in cash, cash equivalents and restricted cash	(6.4)	0.4
Cash, cash equivalents and restricted cash, beginning of period	(3.2)	(1.1)
Cash, cash equivalents and restricted cash, end of period	$ (9.6)	$ (0.7)