Fair Value Measurements (Narrative) (Details) shares in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) Securities	Dec. 31, 2017 USD ($) Securities	Mar. 31, 2017 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	$ 34,258,000,000	$ 33,742,100,000
Trading account securities	8,900,000	500,000
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	1,426	1,436
Available for Sale	$ 28,600,000,000	$ 28,000,000,000
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	1,426	1,436
Trading account securities	$ 8,900,000	$ 500,000
Level 3 | Fair Value, Measurements, Nonrecurring | Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	9,700,000	12,200,000	$ 28,700,000
Level 3 | Fair Value, Measurements, Nonrecurring | OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 400,000	$ 300,000	$ 600,000