Income Tax Expense - NOL and Tax Credit Carryforwards (Details) - USD ($) $ in Millions		Dec. 31, 2018	Dec. 31, 2017
Federal [Member]
NOL and Tax Credit Carryforwards [Line Items]
NOL Carryforwards	[1]	$ 319.0	$ 375.2
Tax Credit Carryforwards		256.4	209.2
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]	305.8	289.9
Tax Credit Carryforwards	[2]	27.4	25.6
Tax Credit Carryforwards, Valuation Allowance		$ 66.0	$ 59.5

[1] Pre-tax amounts.

[2] Net of a $66.0 million valuation allowance as of December 31, 2018 ($59.5 million as of December 31, 2017).