Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash payments :
Interest paid during the year	$ 26.3	$ 22.9	$ 24.3
Income taxes paid during the year	115.3	164.5	217.4
Non cash investing and financing activities :
Changes in accrued capital expenditures	1.6	2.0	4.0
Charitable contribution of marketable securities	13.6	2.0	0.0
Deposits with broker for securities sold short	6.9	8.4	10.8
Securities sold short	$ 5.3	$ 8.2	$ 10.7