Fair Value Measurements (Narrative) (Details) shares in Millions, $ in Millions	Mar. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($) security	Jun. 30, 2016 shares	Jun. 30, 2015 shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale	$ 39,441.7	$ 38,876.3
Trading account securities	$ 1.2	$ 0.3
Visa Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shares issued (in shares) | shares			1.1	1.0
LEVEL 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of available for sale securities portfolio | security	1,771	1,704
Available for sale	$ 34,900.0	$ 34,300.0
LEVEL 2 | External Pricing
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of available for sale securities portfolio | security		1,704
Trading account securities	1.2	$ 0.3
LEVEL 3 | Fair Value, Measurements, Nonrecurring | Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 12.9
Measurement Input, Discount Rate | LEVEL 3 | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average discount factor	0.154	0.153