Emergence from the Chapter 11 Cases and Fresh Start Reporting - Gain on Settlement of Liabilities Subject to Compromise (Details) - USD ($) $ in Millions	Apr. 01, 2017	Sep. 30, 2017	Dec. 31, 2016
Fresh-Start Adjustment [Line Items]
Liabilities subject to compromise	$ 8,416.7		$ 8,440.2
Less amounts issued to settle claims:
Successor Series A Convertible Preferred Stock	(1,305.4)
Issuance of Successor Notes	(1,000.0)
Issuance of Successor Term Loan	(950.0)
Cash payments and accruals for claims and professional fees	(336.4)
Other:
Write-off of Predecessor debt issuance costs, see also (e) below	(18.1)
Total pre-tax gain on plan effects, see also (j) below	3,031.2
Successor
Fresh-Start Adjustment [Line Items]
Liabilities subject to compromise		$ 0.0
Less amounts issued to settle claims:
Successor Common Stock (at par)	(0.7)
Successor Additional paid-in capital	$ (1,774.9)