Fair Value Measurements (Narrative) (Details) shares in Millions	9 Months Ended
	Sep. 30, 2017 USD ($) Securities	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) Securities	Jun. 30, 2016 shares	Jun. 30, 2015 shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for Sale	$ 33,844,900,000		$ 35,579,800,000
Trading account securities	800,000		$ 300,000
Transfers into Level 3 assets	0	$ 0
Transfers out of Level 3 assets	0	0
Transfers into Level 3 liabilities	0	0
Transfers out of Level 3 liabilities	$ 0	0
Visa Class B
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shares issued | 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 | Securities	1,428		1,409
Available for Sale	$ 28,100,000,000		$ 28,100,000,000
Level 2 | External Pricing
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of available for sale securities portfolio | Securities	1,428		1,409
Trading account securities	$ 800,000		$ 300,000
Level 3 | Fair Value, Measurements, Nonrecurring | Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	12,600,000	6,700,000	6,700,000
Level 3 | Fair Value, Measurements, Nonrecurring | OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 1,400,000	$ 1,800,000	$ 700,000