Fair Value Measurements (Narrative) (Details)	6 Months Ended	12 Months Ended
	Jun. 30, 2016 USD ($) Securities shares	Dec. 31, 2015 USD ($) Securities	Jun. 30, 2015 USD ($) shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities portfolio, value	$ 34,632,100,000	$ 32,317,900,000
Trading account securities	3,100,000	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	1,100,000.0		1,000,000.0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of available for sale securities portfolio | Securities	1,298	1,044
Available for sale securities portfolio, value	$ 27,700,000,000	$ 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	1,298	1,044
Trading account securities	$ 3,100,000	$ 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	6,200,000	10,400,000	$ 20,900,000
Fair Value, Measurements, Nonrecurring | Level 3 | OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 1,700,000	$ 3,600,000	$ 300,000