Fair Value Measurements (Narrative) (Details) shares in Millions, $ in Millions	Sep. 30, 2019 USD ($) security	Dec. 31, 2018 USD ($) security	Sep. 30, 2018 USD ($)	Jun. 30, 2016 shares	Jun. 30, 2015 shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale	$ 38,920.2	$ 36,888.8
Trading account securities	$ 1.3	$ 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	1,607	1,479
Available for sale	$ 33,300.0	$ 31,700.0
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		1,479
Trading account securities	1.3	$ 0.3
Level 3 | Fair Value, Measurements, Nonrecurring | Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value	$ 10.5		$ 7.7
Level 3 | Fair Value, Measurements, Nonrecurring | OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value			$ 0.1