Fair Value Measurements - Narrative (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Securities	Dec. 31, 2016 USD ($) Securities shares	Jun. 30, 2016 shares	Dec. 31, 2015 shares	Jun. 30, 2015 shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities portfolio, value	$ 33,742,100,000	$ 35,579,800,000
Trading account securities	500,000	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]
Common stock issued (in shares) | shares		1,100,000.0	1,100,000	1,000,000.0	1,000,000
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of available for sale securities portfolio | Securities	1,436	1,409
Available for sale securities portfolio, value	$ 28,000,000,000	$ 28,100,000,000
Fair Value, Inputs, Level 2 | External Pricing
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	500,000	300,000
Loans | Fair Value, Measurements, Nonrecurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
FAIR VALUE	12,200,000	6,700,000
OREO | Fair Value, Measurements, Nonrecurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
FAIR VALUE	$ 300,000	$ 700,000