Stockholders' Deficit - Summary of Significant Amounts Reclassified (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax benefit, net	$ 1.0	$ 2.0	$ (1.8)
Other comprehensive income (loss), defined benefit plan, gain (loss), reclassification adjustment from AOCI, after tax	3.4	6.6	(6.1)
Net unrealized gains on securities classified as available-for-sale, net of tax
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	0.0	0.0	0.0
Pension and postretirement benefits adjustments, net of tax
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Reclassification from accumulated other comprehensive income, current period, net of tax	(1.7)	(26.2)	(3.5)
Pension and postretirement benefits adjustments, net of tax | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other comprehensive (income) loss, defined benefit plan, prior service cost (credit), reclassification adjustment from AOCI, before tax	0.1	0.1	0.1
Amortization of net actuarial loss	2.1	2.7	4.4
Settlement charge	0.0	31.3	0.0
Total before tax	2.2	34.1	4.5
Tax benefit, net	(0.5)	(7.9)	(1.0)
Other comprehensive income (loss), defined benefit plan, gain (loss), reclassification adjustment from AOCI, after tax	$ 1.7	$ 26.2	$ 3.5