Supplemental Cash Flow Information - Reconciliation of Total Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Supplemental Cash Flow Information
Cash and cash equivalents	$ 12.8	$ 27.2	$ 34.6
Cash, cash equivalents, restricted cash and restricted cash equivalents, end of period	18.4	$ 32.7	39.7	$ 43.3
Prepaid Expenses, Supplies, and Other Current Assets
Supplemental Cash Flow Information
Restricted cash and restricted cash equivalents	0.8		0.7
Other Noncurrent Assets
Supplemental Cash Flow Information
Restricted cash and restricted cash equivalents	$ 4.8		$ 4.4