Income Taxes - Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Income Taxes
Operating loss carryforward, deferred tax asset	$ 29.0
Tax credit carryforward, deferred tax asset	107.2	$ 72.9
State net operating loss carryforwards, deferred tax asset	7.5
Domestic Tax Authority
Income Taxes
Net operating loss carryforward	82.8
Tax credit carryforward	107.2
State and Local Jurisdiction
Income Taxes
Net operating loss carryforward	$ 149.9