Fair Value Measurements (Narrative) (Details) shares in Millions, $ in Millions	Jun. 30, 2021 USD ($) security	Dec. 31, 2020 USD ($) 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	$ 36,864.0	$ 42,022.0
Trading account securities	$ 0.5	$ 0.5
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	2,401	2,260
Available-for-sale	$ 34,300.0	$ 39,200.0
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		2,260
LEVEL 3 | Fair Value, Measurements, Nonrecurring | Measurement Input, Discount Rate | Lower Limit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	0.150	0.150
LEVEL 3 | Fair Value, Measurements, Nonrecurring | Measurement Input, Discount Rate | Upper Limit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	0.200	0.200
LEVEL 3 | Fair Value, Measurements, Nonrecurring | Measurement Input, Discount Rate | Weighted Average
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans, measurement input	0.163	0.168
LEVEL 3 | Fair Value, Measurements, Nonrecurring | Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 8.2