Debt including Capital Lease Obligations - Schedule of Long-term Debt Including Capital Lease Obligations (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Line of Credit Facility
Debt, Long-term and Short-term, Combined Amount	$ 455.0	$ 491.9
Less: debt issuance costs	1.6	3.1
Total debt including capital lease obligations, net of debt issuance costs	453.4	488.8
Less: debt maturing within one year	9.4	7.5
Long-term debt including capital lease obligations	$ 444.0	481.3
Revolving Credit Facility
Line of Credit Facility
Interest Rate	2.81%
Debt, Long-term and Short-term, Combined Amount	$ 0.0	111.9
Letters of credit outstanding, amount	1.8	3.7
Letter of credit remaining amount	$ 548.2	284.4
Term Loan Facility
Line of Credit Facility
Interest Rate	2.82%
Debt, Long-term and Short-term, Combined Amount	$ 375.0	300.0
Capital lease obligations
Line of Credit Facility
Interest Rate	7.67%
Debt, Long-term and Short-term, Combined Amount	$ 80.0	$ 80.0