Fair Value Measurements - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2015 USD ($) Securities shares	Dec. 31, 2014 USD ($) Securities	Jun. 30, 2015 shares
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities portfolio, value	$ 32,317,900,000	$ 29,558,500,000
Trading account securities	1,200,000	4,700,000
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	1,000,000.0		1,000,000.0
OREO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instrument, fair value	$ 3,600,000	4,100,000
Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial Instrument, fair value	$ 10,400,000	$ 14,200,000
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	1,044	881
Available for sale securities portfolio, value	$ 26,100,000,000	$ 25,000,000,000
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	1,044	881
Trading account securities	$ 1,200,000	$ 4,700,000