Accumulated Other Comprehensive Income (Loss) - Changes in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ 1,110.8	$ 1,205.5	$ 1,128.7	$ 1,208.3
Other comprehensive income (loss) before reclassifications			(7.6)
Amounts reclassified from accumulated other comprehensive income (loss)			1.3
Taxes on other comprehensive income (loss)	0.0	0.0	0.0	0.0
Other comprehensive income (loss), net of tax	0.0	(1.4)	(6.3)	(2.1)
Ending balance	1,104.6	1,192.6	1,104.6	1,192.6
Accumulated Other Compre- hensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(55.1)	(52.6)	(48.8)	(51.9)
Other comprehensive income (loss), net of tax		(1.4)	(6.3)	(2.1)
Ending balance	(55.1)	$ (54.0)	(55.1)	$ (54.0)
Employee Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance			(48.0)
Other comprehensive income (loss) before reclassifications			0.0
Amounts reclassified from accumulated other comprehensive income (loss)			1.2
Taxes on other comprehensive income (loss)			0.0
Other comprehensive income (loss), net of tax			1.2
Ending balance	(46.8)		(46.8)
Employee Benefit Plans | Qualified Plans | Pension plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance			(47.4)
Ending balance	(46.2)		(46.2)
Employee Benefit Plans | Qualified Plans | Post-retirement plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance			0.2
Ending balance	0.2		0.2
Employee Benefit Plans | Nonqualified Plans | Non-qualified benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance			(0.8)
Ending balance	(0.8)		(0.8)
Interest Rate Swap
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance			(0.8)
Other comprehensive income (loss) before reclassifications			(7.6)
Amounts reclassified from accumulated other comprehensive income (loss)			0.1
Taxes on other comprehensive income (loss)			0.0
Other comprehensive income (loss), net of tax			(7.5)
Ending balance	$ (8.3)		$ (8.3)