Income Taxes - Gross Carryforwards (Details) $ in Millions	Dec. 31, 2017 USD ($)
Foreign Tax Authority
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 1,389.0	[1]
Operating loss carryforwards, valuation allowance	861.6
Domestic Tax Authority
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	82.6	[1]
Tax credit carryforwards, valuation allowance	82.6
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	248.5	[1]
Operating loss carryforwards, valuation allowance	203.6
Tax credit carryforwards	71.9	[1]
Tax credit carryforwards, valuation allowance	71.9
Capital Loss Carryforward | Foreign Tax Authority
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	22.0	[1]
Tax credit carryforwards, valuation allowance	22.0
Interest Carryforward
Operating Loss Carryforwards [Line Items]
Tax credit carryforwards	478.8	[1]
Tax credit carryforwards, valuation allowance	$ 127.0

[1] Utilization of such carryforwards within the applicable statutory periods is uncertain.