Condensed Consolidated Statement of Cash Flows - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Cash flows — operating activities:
Net income	$ 48.5	$ 273.8	$ 370.9	$ 384.0
Less: income from discontinued operations	(0.3)	222.8	4.5	260.0
Income from continuing operations	48.8	51.0	366.4	124.0
Depreciation and amortization	34.7	22.8	92.7	69.1	$ 96.0	$ 91.1
Net gains on investments			(151.9)	(2.8)
Gain recognized on step-up of unconsolidated affiliates			(43.8)	0.0
Amortization of share-based compensation			31.2	11.0
Equity in earnings of unconsolidated affiliates	(4.2)	(7.0)	(27.3)	(23.9)	(27.2)	(45.4)
Cash dividends from unconsolidated affiliates			1.8	0.3
Deferred income taxes			(7.2)	(0.1)
Changes in accounts receivable			23.1	1.3
Changes in other assets			(61.8)	(0.9)
Changes in client funds obligations			(16.2)	7.5
Changes in client funding receivable			16.2	(7.5)
Changes in accounts payable and accrued liabilities			(81.5)	11.6
Changes in income taxes payable			3.9	8.2
Changes in deferred revenues and gains			(33.1)	(10.0)
Changes in accrued compensation and benefits			(16.0)	(32.4)
Other, net			1.7	(7.8)
Net cash provided from continuing operating activities			98.2	147.6
Net cash provided from (used in) discontinued operating activities			(12.1)	14.1
Net cash provided from operating activities			86.1	161.7
Cash flows — investing activities:
Cash paid for capital expenditures			(61.7)	(42.7)
Investments in securities			(51.6)	(198.3)
Proceeds from (advances to) unconsolidated affiliates			31.7	(25.0)
Proceeds from sales/maturities of investments			87.3	257.5
Net change in funds held to satisfy client funds obligations			(42.4)	167.9
Proceeds from sale of properties			0.0	5.7
Acquisition of businesses, net of cash and cash equivalents acquired			(38.9)	(93.5)
Proceeds from sale of business, net of cash and cash equivalents sold			0.8	9.5
Other, net			0.5	1.0
Net cash provided from (used in) continuing investing activities			(74.3)	82.1
Net cash provided from discontinued investing activities			40.6	409.4
Net cash provided from (used in) investing activities			(33.7)	491.5
Cash flows — financing activities:
Proceeds from issuance of common stock			2.4	4.6
Principal payments on debt			(106.8)	(4.9)
Net borrowings (repayments) on revolving credit facilities			250.0	(216.1)
Net borrowings (repayments) on accounts receivable securitization program			(37.6)	88.6
Net change in client funds obligations			42.4	(167.9)
Common stock repurchased			(235.4)	(240.7)
Payment of cash dividends			(33.0)	(32.8)
Excess tax benefits from share-based compensation			0.0	3.7
Receipt of third party capital in investment fund			0.8	6.7
Net cash used in continuing financing activities			(117.2)	(558.8)
Net cash used in discontinued financing activities			(0.2)	0.0
Net cash used in financing activities			(117.4)	(558.8)
Effect of exchange rate on cash and cash equivalents			6.2	0.0
Net increase (decrease) in cash and cash equivalents, including cash within assets held for sale			(58.8)	94.4
Cash and cash equivalents, beginning of period			199.5	89.6	89.6
Cash and cash equivalents, end of period	140.7	184.0	140.7	184.0	199.5	$ 89.6
Less: cash and cash equivalents held for sale	0.0	3.1	0.0	3.1
Cash and cash equivalents of continuing operations, end of period	$ 140.7	$ 180.9	$ 140.7	$ 180.9	$ 195.5