Commitments and Contingencies Commitments and Contingencies - Long-term Purchase Commitments (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Long-term Purchase Commitment [Line Items]
Long-term purchase commitments	$ 667.7
2018
Long-term Purchase Commitment [Line Items]
Long-term purchase commitments	151.8
2019
Long-term Purchase Commitment [Line Items]
Long-term purchase commitments	86.4
2020
Long-term Purchase Commitment [Line Items]
Long-term purchase commitments	43.0
2021
Long-term Purchase Commitment [Line Items]
Long-term purchase commitments	37.9
2022
Long-term Purchase Commitment [Line Items]
Long-term purchase commitments	38.0
Thereafter
Long-term Purchase Commitment [Line Items]
Long-term purchase commitments	$ 310.6