Consolidated Condensed Statements of Comprehensive Income (Unaudited) - USD ($) $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2016		Sep. 30, 2015		Sep. 30, 2016		Sep. 30, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net income		$ 69.4		$ 53.2		$ 148.5		$ 123.0
Other comprehensive income, net of tax
Gain on cash flow hedges		0.2		0.2		0.6		3.3
Amortization of unrecognized benefit costs and other	[1]	0.5		0.2		0.8		1.8
Recognized cost due to curtailment		0.0	[2]	0.0		0.1	[2]	0.0
Change in benefit obligation due to remeasurement/ valuation		(6.9)		(5.7)		(6.9)		(5.7)
Recognized cost due to settlement		0.0		7.7	[3]	0.0		7.7	[3]
Other comprehensive income (loss), net of tax		(6.2)		2.4		(5.4)		7.1
Comprehensive income		$ 63.2		$ 55.6		$ 143.1		$ 130.1

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

[2] Related to the estimated curtailment loss for the SERP. See Note 5 for additional information.

[3] Related to the settlement of the TECO Coal black lung obligation at the closing of the sale. See Notes 15 for additional information.