Basis of Presentation and Significant Accounting Policies - Estimated Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Derivative instruments	$ (29.4)
Convertible senior notes
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	98.2	$ 271.3
Senior notes due 2022
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	1,359.8
Senior notes due 2021
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	485.9
Senior notes due 2019
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	877.5	527.5
Euro private placement notes due 2020
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	13.6
Level 1
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Derivative instruments	0.0
Level 1 | Convertible senior notes
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	98.2	271.3
Level 1 | Senior notes due 2022
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	1,359.8
Level 1 | Senior notes due 2021
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	0.0
Level 1 | Senior notes due 2019
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	877.5	527.5
Level 1 | Euro private placement notes due 2020
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	0.0
Level 2
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Derivative instruments	(29.4)
Level 2 | Convertible senior notes
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	0.0	0.0
Level 2 | Senior notes due 2022
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	0.0
Level 2 | Senior notes due 2021
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	485.9
Level 2 | Senior notes due 2019
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	0.0	0.0
Level 2 | Euro private placement notes due 2020
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	13.6
Level 3
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Derivative instruments	0.0
Level 3 | Convertible senior notes
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	0.0	0.0
Level 3 | Senior notes due 2022
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	0.0
Level 3 | Senior notes due 2021
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	0.0
Level 3 | Senior notes due 2019
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	0.0	0.0
Level 3 | Euro private placement notes due 2020
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	0.0
Money market funds
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Money market funds	376.4	330.8
Money market funds | Level 1
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Money market funds	376.4	330.8
Money market funds | Level 2
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Money market funds	0.0	0.0
Money market funds | Level 3
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Money market funds	0.0	0.0
Carrying Value
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Derivative instruments	(29.4)
Carrying Value | Convertible senior notes
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	59.1	91.9
Carrying Value | Senior notes due 2022
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	1,600.0
Carrying Value | Senior notes due 2021
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	560.2
Carrying Value | Senior notes due 2019
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	914.0	500.0
Carrying Value | Euro private placement notes due 2020
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Senior notes	14.9
Carrying Value | Money market funds
Basis Of Presentation And Summary Of Significant Accounting Policies [Line Items]
Money market funds	$ 376.4	$ 330.8