Stockholders' Deficit - Summary of Changes To Accumulated Other Comprehensive Loss By Component (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Amounts Recognized in Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	$ (63.5)
Other comprehensive income (loss), net of tax	5.1	$ 19.1	$ (4.5)
Cumulative effect of adoption of ASU 2018-02		0.0
Ending balance	(58.4)	(63.5)
Net unrealized gains on securities classified as available-for-sale, net of tax
Schedule of Amounts Recognized in Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	1.6	1.9	2.2
Other comprehensive (loss) income before reclassification	(0.4)	(0.3)	(0.3)
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0	0.0
Other comprehensive income (loss), net of tax	(0.4)	(0.3)	(0.3)
Cumulative effect of adoption of ASU 2018-02		0.0
Ending balance	1.2	1.6	1.9
Cumulative non-U.S. dollar translation adjustments, net of tax
Schedule of Amounts Recognized in Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(28.1)	(24.2)	(10.4)
Other comprehensive (loss) income before reclassification	7.2	(0.2)	(13.8)
Amounts reclassified from accumulated other comprehensive loss	0.0	0.0	0.0
Other comprehensive income (loss), net of tax	7.2	(0.2)	(13.8)
Cumulative effect of adoption of ASU 2018-02		(3.7)
Ending balance	(20.9)	(28.1)	(24.2)
Pension and postretirement benefits adjustments, net of tax
Schedule of Amounts Recognized in Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(37.0)	(45.2)	(54.8)
Other comprehensive (loss) income before reclassification	(3.4)	(6.6)	6.1
Amounts reclassified from accumulated other comprehensive loss	(1.7)	(26.2)	(3.5)
Other comprehensive income (loss), net of tax	(1.7)	19.6	9.6
Cumulative effect of adoption of ASU 2018-02		(11.4)
Ending balance	(38.7)	(37.0)	(45.2)
Accumulated Other Comprehensive Loss
Schedule of Amounts Recognized in Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance	(63.5)	(67.5)	(63.0)
Other comprehensive (loss) income before reclassification	3.4	(7.1)	(8.0)
Amounts reclassified from accumulated other comprehensive loss	(1.7)	(26.2)	(3.5)
Other comprehensive income (loss), net of tax	5.1	19.1	(4.5)
Cumulative effect of adoption of ASU 2018-02		(15.1)
Ending balance	(58.4)	(63.5)	(67.5)
Retained Loss
Schedule of Amounts Recognized in Other Comprehensive Income (Loss) [Roll Forward]
Other comprehensive income (loss), net of tax	$ 0.0	0.0	$ 0.0
Cumulative effect of adoption of ASU 2018-02		$ 15.1