New Accounting Pronouncements (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2020
Cumulative effect of adoption
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member	us-gaap:AccountingStandardsUpdate201409Member
Total stockholders’ equity		$ 1,851.9	$ 731.2	$ (73.3)	$ 2,655.8
Retained Earnings (Accum- ulated Deficit)
Cumulative effect of adoption
Total stockholders’ equity		161.6	(968.0)	(1,051.9)	$ (507.3)
Cumulative Effect, Period of Adoption, Adjustment
Cumulative effect of adoption
Total stockholders’ equity		0.6	1,085.7	(9.6)
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings (Accum- ulated Deficit)
Cumulative effect of adoption
Total stockholders’ equity		$ 0.6	$ 1,085.7	$ (9.6)