CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) shares in Millions, $ in Millions	Total	Preferred Stock	Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Loss	Noncontrolling Interests
Beginning Balance at Dec. 31, 2016	$ 2,816.4		$ 0.8	$ 1,166.5	$ 2,006.1	$ (367.6)	$ 10.6
Beginning Balance (in shares) at Dec. 31, 2016		0.7	85.2
Net income	43.0				42.4		0.6
Currency translation adjustment	80.5					79.3	1.2
Change in qualifying cash flow hedge, net of tax	1.3					1.3
Issuance of common stock for exercise of options and employee stock purchases, net of shares withheld for tax	17.3		$ 0.1	17.2
Issuance of common stock for exercise of options and employee stock purchases, net of shares withheld for tax, (in shares)			0.3
Purchase and cancellation of convertible preferred stock	(34.2)			(51.5)	17.3
Preferred stock dividend	(36.3)				(36.3)
Stock compensation expense	21.3			21.3
Reduction to noncontrolling interests due to additional ownership	(0.7)			(0.3)			(0.4)
Ending Balance at Dec. 31, 2017	2,908.6		$ 0.9	1,153.2	2,029.5	(287.0)	12.0
Ending Balance (in shares) at Dec. 31, 2017		0.7	85.5
Net income	50.1				50.2		(0.1)
Currency translation adjustment	(59.9)					(59.3)	(0.6)
Change in qualifying cash flow hedge, net of tax	0.5					0.5
Issuance of common stock for exercise of options and employee stock purchases, net of shares withheld for tax	8.4			8.4
Issuance of common stock for exercise of options and employee stock purchases, net of shares withheld for tax, (in shares)			0.2
Purchase and cancellation of convertible preferred stock	(14.8)			(29.3)	14.5
Purchase and cancellation of convertible preferred stock (in shares)		(0.1)
Preferred stock dividend	(17.1)				(17.1)
Stock compensation expense	12.8			12.8
Cumulative effect of new accounting standard (see Note 2)	13.0				13.0
Ending Balance at Jun. 30, 2018	$ 2,901.6		$ 0.9	$ 1,145.1	$ 2,090.1	$ (345.8)	$ 11.3
Ending Balance (in shares) at Jun. 30, 2018		0.6	85.7