Fair Value Measurements (Narrative) (Details) shares in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($) Securities shares	Dec. 31, 2015 USD ($) Securities	Mar. 31, 2015 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities portfolio, value	$ 33,330,200,000	$ 32,317,900,000
Trading account securities	200,000	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]
Number of common stock shares sold | shares	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	1,154	1,044
Available for sale securities portfolio, value	$ 26,500,000,000	$ 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	1,152	1,044
Trading account securities	$ 200,000	$ 1,200,000
Level 2 | Market Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of available for sale securities portfolio | Securities	2
Available for sale securities portfolio, value	$ 2,100,000
Fair Value, Measurements, Nonrecurring | Level 3 | Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	10,100,000	10,400,000	$ 23,100,000
Fair Value, Measurements, Nonrecurring | Level 3 | OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 2,500,000	$ 3,600,000	$ 300,000