Commitments (Summary of Commitments) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Financing assets - Due to Expire Within One Year	$ 1,793.8
Financing assets - Due to Expire After One Year	4,811.8
Financing assets - Total Outstanding	6,605.6	$ 6,008.1
Standby letters of credit - Due to Expire Within One Year	55.1
Standby letters of credit - Due to Expire After One Year	191.0
Standby letters of credit - Total Outstanding	246.1	232.2
Other letters of credit - Due to Expire Within One Year	26.7
Other letters of credit - Total Outstanding	26.7	14.0
Deferred purchase credit protection agreements - Due to Expire Within One Year	1,494.1
Deferred purchase credit protection agreements - Total Outstanding	1,494.1	2,060.5
Guarantees, acceptances and other recourse obligations - Due to Expire Within One Year	1.1
Guarantees, acceptances and other recourse obligations - Total Outstanding	1.1	1.6
Aerospace purchase commitments - Due to Expire Within One Year	0.0
Aerospace purchase commitments - Due to Expire After One Year	0.0
Aerospace purchase commitments - Total Outstanding	0.0	8,683.5
Rail and other purchase commitments - Due to Expire Within One Year	236.1
Rail and other purchase commitments - Due to Expire After One Year	33.2
Rail and other purchase commitments - Total Outstanding	$ 269.3	$ 300.7