Summary of Significant Accounting Policies - Property and Depreciation (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Property and Depreciation
Assets in which units-of-production method of depreciation is used	$ 66.6
Buildings
Property and Depreciation
Weighted average useful lives	20 years
Land improvements
Property and Depreciation
Weighted average useful lives	12 years
Machinery and equipment
Property and Depreciation
Weighted average useful lives	9 years 6 months