Business Segments - Geography, Segment Reporting (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	$ 1,893.2	$ 1,589.1	$ 1,319.1
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	1,138.6	938.4	808.9
Long-lived assets	396.0	370.0	395.1
Iceland [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	754.6	650.7	510.2
Long-lived assets	554.3	583.0	625.9
Other [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 76.5	$ 75.0	$ 78.7