Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2016
Income Tax Contingency [Line Items]
Deferred tax benefit from tax cuts and jobs act	$ 11.6
Limit on net operating loss usage, per annum	265.0
Net operating loss (NOL) carry forwards	877.3	$ 2,528.3
Deferred tax assets, valuation allowance	280.6	278.4
Deferred income tax liability	61.0
Reduction in deferred income tax liabilities	967.0
Reduction in deferred income tax liabilities, sale of commercial air business	964.0
Reduction in deferred income tax liabilities, U.S. tax reform	13.6
Reduction in uncertain tax positions	28.3
Reductions for tax positions of prior years	10.6
Reductions for disposal	6.6
Recognized increase in interest and penalties associated with uncertain tax positions	5.4
Accrual for interest and penalties	6.3
Unrecognized tax benefits	19.8	48.1
Potential decrease to tax benefits, minimum	0.0
Potential decrease to tax benefits, maximum	5.0
U.S. Federal
Income Tax Contingency [Line Items]
Net operating loss (NOL) carry forwards	2,400.0
Deferred tax asset	504.0
Operating loss carryforwards	2,400.0
Deferred tax assets no longer subject to limitation/adjustment	1,400.0
Deferred tax assets, valuation allowance		690.0	$ 647.0
Operating loss carryforwards, valuation allowance	0.0
U.S. Federal | Pre-Emergence Bankruptcy
Income Tax Contingency [Line Items]
Operating loss carryforwards	1,000.0
U.S. State
Income Tax Contingency [Line Items]
Deferred tax asset	322.0
Operating loss carryforwards	6,100.0
State NOLs expiring in next fiscal year	188.0
Operating loss carryforwards, valuation allowance	208.6	250.0
Foreign Tax Authority
Income Tax Contingency [Line Items]
Deferred tax asset	51.1
Operating loss carryforwards	203.8
Deferred tax assets, valuation allowance	32.4
Deferred tax asset valuation, change in amount		$ 38.9
Reductions for tax positions of prior years	21.0
Capital Loss Carryforward | U.S. Federal
Income Tax Contingency [Line Items]
Capital loss carryforwards, valuation allowance	33.2
Capital Loss Carryforward | U.S. State
Income Tax Contingency [Line Items]
Capital loss carryforwards, valuation allowance	$ 6.4