Income Tax Expense - NOL and Tax Credit Carryforwards (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Federal [Member]
NOL and Tax Credit Carryforwards [Line Items]
NOL Carryforwards	[1]	$ 485.3	$ 493.0
Tax Credit Carryforwards		163.7	113.6
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]	294.4	228.6
Tax Credit Carryforwards	[2]	21.0	20.0
Tax Credit Carryforwards, Valuation Allowance		$ 42.7	$ 31.2

[1] Pre-tax amounts.

[2] Net of a $42.7 million valuation allowance as of December 31, 2016 ($31.2 million as of December 31, 2015).