Statement of Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted Cash and Cash Equivalents Items [Line Items]
Income taxes paid	$ 0.8	$ 1.5	$ 0.3
Income tax refunds	0.0	5.1	$ 1.0
Seismic Operations Bonds [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash and cash equivalents	$ 0.6	$ 0.5