Income Tax Expense - NOL and Tax Credit Carryforwards (Details) - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014
Federal [Member]
NOL and Tax Credit Carryforwards [Line Items]
NOL Carryforwards	[1]	$ 493.0	$ 413.7
Tax Credit Carryforwards		113.6	59.3
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]	228.6	184.7
Tax Credit Carryforwards	[2]	20.0	14.7
Tax Credit Carryforwards, Valuation Allowance		$ 31.2	$ 21.7

[1] Pretax amounts

[2] Net of a $31.2 million valuation allowance as of December 31, 2015 ($21.7 million as of December 31, 2014).