Commitments (Summary of Commitments) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Financing assets - Due to Expire Within One Year	$ 3,477.0
Financing assets - Due to Expire After One Year	4,893.9
Financing assets - Total Outstanding	8,370.9	$ 6,459.7
Standby letters of credit - Due to Expire Within One Year	52.5
Standby letters of credit - Due to Expire After One Year	178.8
Standby letters of credit - Total Outstanding	231.3	199.6
Other letters of credit - Due to Expire Within One Year	7.3
Other letters of credit - Due to Expire After One Year	3.1
Other letters of credit - Total Outstanding	10.4	6.7
Deferred purchase credit protection agreements - Due to Expire Within One Year	1,556.1
Deferred purchase credit protection agreements - Total Outstanding	1,556.1	2,060.6
Rail and other purchase commitments - Due to Expire Within One Year	690.6
Rail and other purchase commitments - Total Outstanding	$ 690.6	$ 813.7