Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 32.9	$ 65.0	$ 115.7	$ 113.8
Other comprehensive income:
Amortization of net actuarial losses and prior service credits, net of income taxes of $2.2 million, $4.3 million, $6.8 million, and $12.7 million	6.9	12.3	20.9	37.3
Comprehensive income	$ 39.8	$ 77.3	$ 136.6	$ 151.1