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Income Tax Expense - NOL and Tax Credit Carryforwards (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Federal [Member]    
NOL and Tax Credit Carryforwards [Line Items]    
NOL Carryforwards [1] $ 375.2 $ 485.3
Tax Credit Carryforwards 209.2 163.7
Tax Credit Carryforwards, Valuation Allowance 0.0  
NOL Carryforwards, Valuation Allowance 0.0  
State [Member]    
NOL and Tax Credit Carryforwards [Line Items]    
NOL Carryforwards [1] 289.9 294.4
Tax Credit Carryforwards [2] 25.6 21.0
Tax Credit Carryforwards, Valuation Allowance $ 59.5 $ 42.7
[1] Pre-tax amounts.
[2] Net of a $59.5 million valuation allowance as of December 31, 2017 ($42.7 million as of December 31, 2016).