Fair Value Measurements - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) Securities shares	Dec. 31, 2015 USD ($) Securities shares	Jun. 30, 2016 shares	Jun. 30, 2015 shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities portfolio, value	$ 35,579,800,000	$ 32,317,900,000
Trading account securities	300,000	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]
Common stock issued (in shares) | shares	1,100,000.0	1,000,000.0	1,100,000	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	1,409	1,044
Available for sale securities portfolio, value	$ 28,100,000,000	$ 26,100,000,000
Fair Value, Inputs, 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	1,409	1,044
Trading account securities	$ 300,000	$ 1,200,000
Loans | Fair Value, Measurements, Nonrecurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	6,700,000	10,400,000
OREO | Fair Value, Measurements, Nonrecurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	$ 700,000	$ 3,600,000