Redeemable Noncontrolling Interest (Details) $ in Millions		3 Months Ended			6 Months Ended			12 Months Ended
		Jun. 30, 2018 USD ($) shares	Dec. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) shares		Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Apr. 01, 2021
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance, Beginning of period					$ 503.7	[1]	$ 483.7	$ 483.7
Net income attributable to NCI		$ (3.0)		$ 5.1	8.8		11.1	5.9
Other comprehensive income (loss) attributable to NCI		(0.6)		1.3	(4.6)		2.2	2.3
Exercise of employee stock options					9.4		3.4	3.4
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests		(295.9)	$ 0.0	0.0
Change in redemption value of NCI					(3.2)		1.4	8.4
Balance, End of period	[1]	$ 218.2	$ 503.7	$ 501.8	$ 218.2		$ 501.8	$ 503.7
ARX Holding Corp. [Member]
Redeemable Noncontrolling Interest [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage		86.70%	69.00%	69.00%	86.70%		69.00%	69.00%
Shares held in Employee Stock Option Plan, Suspense Shares | shares		16,067			16,067
Employee Stock Ownership Plan (ESOP), Deferred Shares, Fair Value		$ 23.2			$ 23.2
Number of Shares Put Pursuant to Stockholders' Agreement | shares		204,527			204,527
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period | shares		5,483
Payments to Acquire Interest in Subsidiaries and Affiliates		$ 295.9
Scenario, Forecast [Member] | ARX Holding Corp. [Member]
Redeemable Noncontrolling Interest [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage									100.00%

[1] See Note 12 – Redeemable Noncontrolling Interest for further discussion.