Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 137.7	$ 141.0	$ 137.3
Other comprehensive income:
Amortization of net actuarial losses, net of $13.4 million, $7.6 million, and $16.8 million of income taxes in 2020, 2019, and 2018, respectively	40.3	29.5	49.7
Actuarial losses, net of $10.4 million, $6.9 million, and $5.5 million of income taxes in 2020, 2019, and 2018, respectively	(31.5)	(26.5)	(16.4)
Amortization of prior service costs, net of income taxes	0.1	(0.1)	(0.1)
Other comprehensive income, net of income taxes	8.9	2.9	33.2
Comprehensive income	$ 146.6	$ 143.9	$ 170.5