Fair Value Measurements - Commodity and Foreign Currency Contracts (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Fair value measurement, additional disclosure
Cash collateral deposits held by financial counterparties	$ 47.5	$ 174.6
Cash collateral deposits held by financial counterparties, offset against liabilities	20.0	132.2
Cash collateral deposits held by financial counterparties, not offset	27.6	42.4
Cash collateral received from customers	1.0	21.6
Commodity contracts
Derivative instruments, at their respective fair value positions
Total net assets	43.3	231.5
Total net liabilities	37.0	74.1
Commodity contracts | Short-term derivative assets, net
Derivative instruments, at their respective fair value positions
Total net assets	28.1	212.6
Commodity contracts | Identifiable intangible and other non-current assets
Derivative instruments, at their respective fair value positions
Total net assets	15.2	18.9
Commodity contracts | Accrued expenses and other current liabilities
Derivative instruments, at their respective fair value positions
Total net liabilities	31.9	68.7
Commodity contracts | Other long-term liabilities
Derivative instruments, at their respective fair value positions
Total net liabilities	5.2	5.4
Foreign currency contracts
Derivative instruments, at their respective fair value positions
Total net assets	7.0	8.3
Total net liabilities	1.9	0.9
Foreign currency contracts | Short-term derivative assets, net
Derivative instruments, at their respective fair value positions
Total net assets	5.7	7.6
Foreign currency contracts | Identifiable intangible and other non-current assets
Derivative instruments, at their respective fair value positions
Total net assets	1.3	0.7
Foreign currency contracts | Accrued expenses and other current liabilities
Derivative instruments, at their respective fair value positions
Total net liabilities	$ 1.9	$ 0.9