Income Taxes - Gross Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Loss and credit carryforwards	$ 1,556.6	$ 1,755.1
Loss and credit carryforwards, tax effected	359.2	292.5
Tax credit carryforwards, valuation allowance	343.7	285.2
Research Tax Credit Carryforward
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	58.8	58.9
Interest Carryforward
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	331.9	131.0
Tax credit carryforwards, tax effected	76.1	$ 30.5
Expire Through Tax Year 2038
Operating Loss Carryforwards [Line Items]
Loss and credit carryforwards	469.0
Loss and credit carryforwards, tax effected	$ 129.2