Accounting for Asset Retirement Obligations Other Commitments (Details) - USD ($) $ in Millions		9 Months Ended	29 Months Ended
		Dec. 31, 2014	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015	Jul. 31, 2014	Mar. 17, 2014
Other Commitments [Line Items]
Restricted cash	[1]		$ 31.3		$ 851.4
Asset retirement obligations			747.9		$ 749.7
Plant City Trust [Member]
Other Commitments [Line Items]
Restricted cash							$ 189.2
Increase in restricted cash		$ 8.7
Assets held-in-trust, current				$ 200.0
Plant City Trust [Member] | Surety Bond [Member]
Other Commitments [Line Items]
Post Closure And Environmental Costs				$ 260.0
Bonnie Facility Trust [Member]
Other Commitments [Line Items]
Restricted cash						$ 14.5
Increase in restricted cash			6.0
Plant City and Bonnie Facilities [Member]
Other Commitments [Line Items]
Asset retirement obligations			$ 93.5

[1] Included in restricted cash, as of December 31, 2015, was approximately $200 million that was released to us following our replacement of the financial assurance provided by this cash with a surety bond in 2016, and approximately $630 million that was placed in trust in August 2016, following the effectiveness of the consent decrees discussed under "EPA RCRA Initiative" in Note 13 of our Notes to Consolidated Financial Statements. The funds have been invested in marketable securities as discussed in Note 11 of our Consolidated Financial Statements.