Consolidated Condensed Statements of Comprehensive Income (Unaudited) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 58.0		$ 50.1
Other comprehensive income, net of tax
Gain on cash flow hedges	0.3		0.2
Amortization of unrecognized benefit costs	0.6	[1]	0.5	[1]
Increase in unrecognized postemployment costs	0		(8.2)
Other comprehensive income (loss), net of tax	0.9		(7.5)
Comprehensive income	$ 58.9		$ 42.6

[1] Related to postretirement and postemployment benefits. See Note 5 for additional information.