Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred income taxes	$ 0.9	$ 0.9	$ 0.9	$ 0.9
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning of year	1,768.1
End of year	1,849.3	1,768.1
Pension & other benefit costs
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning of year	(2.2)	(2.1)	(2.2)
Change during the year	0.0	(0.1)	0.1
End of year	(2.2)	(2.2)	(2.1)
Accumulated other comprehensive income (loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning of year	(1.3)	(1.2)
Change during the year	0.0	(0.1)	0.1
End of year	$ (1.3)	$ (1.3)	$ (1.2)