Fair Value Measurements (Narrative) (Details) shares in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($) Securities	Dec. 31, 2015 USD ($) Securities	Jun. 30, 2016 shares	Sep. 30, 2015 USD ($)	Jun. 30, 2015 shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for Sale	$ 35,973,600,000	$ 32,317,900,000
Trading account securities	800,000	1,200,000
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	1,360	1,044
Available for Sale	$ 28,300,000,000	$ 26,100,000,000
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	1,360	1,044
Trading account securities	$ 800,000	$ 1,200,000
Fair Value, Measurements, Nonrecurring | Level 3 | Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,700,000	10,400,000		$ 10,900,000
Fair Value, Measurements, Nonrecurring | Level 3 | OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 1,800,000	$ 3,600,000		$ 300,000