Statement of Cash Flow Information - Narrative (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items
Income taxes paid	$ 4.2	$ 0.7
Income tax refunds	0.5
Seismic Operations Bonds [Member]
Restricted Cash and Cash Equivalents Items
Restricted cash	$ 0.1		$ 0.6