Pension, Postretirement and Postemployment Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of net periodic benefit cost (recognized in our consolidated statements of income):
Payment amount		$ 10.3
Settlement		3.0
Pension plan | Pool Pension Plan
Funded status:
unfunded liability	$ 6.7	7.3
Components of net periodic benefit cost (recognized in our consolidated statements of income):
Net periodic benefit cost	$ 1.2	$ 1.1	$ 1.0