Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating Loss Carryforwards [Line Items]
Effective income tax rate, percentage	18.50%	6.10%	26.20%	24.10%
Acquired in-process research and development	$ 30.0	$ 205.0	$ 1,654.5	$ 1,062.6
ARMO and AurKa
Operating Loss Carryforwards [Line Items]
Acquired in-process research and development			$ 1,560.0
Tax Year 2013
Operating Loss Carryforwards [Line Items]
Tax year under examination			2013
Tax Year 2014
Operating Loss Carryforwards [Line Items]
Tax year under examination			2014
Tax Year 2015
Operating Loss Carryforwards [Line Items]
Tax year under examination			2015