Basis of Presentation - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions				6 Months Ended	9 Months Ended	12 Months Ended
	Aug. 07, 2018 USD ($) shares	Aug. 06, 2018 $ / shares shares	Jul. 19, 2018	Jun. 30, 2018 USD ($)	Sep. 30, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares	Jul. 12, 2018 $ / shares
Subsidiary, Sale of Stock [Line Items]
Ordinary shares, nominal value per share (in dollars per share) | $ / shares					$ 0.10	$ 10	$ 0.01
Stock split ratio			0.1
Tax Act, net benefit					$ 93.1
Net income tax benefit increase				$ 10.0	32.2
Provision (benefit) from income taxes					0.7
Tax Act, increase in the tax benefit					124.9	$ 124.2
Tax Act, decrease in tax expense					31.5
Effective income tax rate reconciliation, repatriation of foreign earnings					31.8	$ 63.3
Tax Act, income tax expense					8.1
Tax Act, decrease in income tax expense				$ 3.4
Tax Act, tax liability					$ 6.0
IPO
Subsidiary, Sale of Stock [Line Items]
Shares issued (in shares) | shares		51.8
Sale of stock, price per share (in dollars per share) | $ / shares		$ 17.00
Concurrent Private Placement
Subsidiary, Sale of Stock [Line Items]
Shares issued (in shares) | shares	10.6
Sale of stock, price per share (in dollars per share) | $ / shares		$ 17.00
IPO and Current Private Placement
Subsidiary, Sale of Stock [Line Items]
Net proceeds, IPO	$ 1,000.0