Emergence from the Chapter 11 Cases and Fresh Start Reporting - Reorganization Items, Net (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended	9 Months Ended
	Apr. 01, 2017	Apr. 01, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fresh-Start Adjustment [Line Items]
Gain on settlement of claims (per above)	$ (3,031.2)
Professional fees	336.4
Payments of Reorganization Professional Fees	8.3			$ 250.0
Income Tax Effect Related to Predecessor Deferred Tax Liabilities
Fresh-Start Adjustment [Line Items]
Fresh start income tax adjustments, net	(177.8)
Income Tax Effect Related to AOCI
Fresh-Start Adjustment [Line Items]
Fresh start income tax adjustments, net	(81.5)
Income Tax Effect Related to Unrecognized Tax Benefits
Fresh-Start Adjustment [Line Items]
Fresh start income tax adjustments, net	(6.7)
Income Tax Effect Related to Predecessor Deferred Tax Assets of Discontinued Operations
Fresh-Start Adjustment [Line Items]
Fresh start income tax adjustments, net	$ 12.1
Predecessor
Fresh-Start Adjustment [Line Items]
Gain on settlement of claims (per above)		$ (3,031.2)	$ 0.0		$ 0.0
Fresh start adjustments, net (per above)		3,363.1	0.0		0.0
Fresh start income tax adjustments, net		(253.9)	0.0		0.0
Loss on termination of derivative contracts		0.0	0.0		75.2
Professional fees		42.5	31.1		52.7
Accounts payable settlement gains		(0.7)	(0.5)		(0.7)
Interest income		(0.4)	(0.9)		(1.1)
Other		0.0	0.0		(1.0)
Reorganization items, net		627.2	29.7		125.1
Cash paid for “Reorganization items, net”		$ 45.8	$ 30.7		$ 30.7