Statement of Cash Flow Information - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items
Income taxes paid	$ 3.5	$ 0.3
Income tax refunds	0.1
Seismic Operations Bonds [Member]
Restricted Cash and Cash Equivalents Items
Restricted cash	$ 0.1		$ 0.6