New Accounting Pronouncements (Details) - USD ($) $ in Millions	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements
Total equity	$ 5,105.2	$ 4,800.3	$ 4,596.2	$ 4,211.1	$ 4,203.2	$ 4,064.2
Accounting Standards Update 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements
Total equity			(3.8)
Retained Earnings
New Accounting Pronouncements
Total equity	$ 3,587.1	$ 3,419.4	3,348.4	$ 3,158.7	$ 3,104.6	$ 3,028.7
Retained Earnings | Accounting Standards Update 2016-13 [Member] | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements
Total equity			$ (3.8)