Carryforwards (Details 5) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
State and local jurisdiction [Member]
Carryforwards [Line Items]
Operating loss carryforwards	$ 130.1	$ 114.7
Investment tax credit carryforward, amount	28.0	20.7
Federal and State Taxing Authority [Member]
Carryforwards [Line Items]
Tax credit carryforward, amount	$ 52.5	$ 43.3