Fair Value Measurements - Additional Information (Detail) (USD $)	1 Months Ended	9 Months Ended
	Apr. 30, 2014	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Available for sale securities portfolio, value		$ 28,107,700,000		$ 28,392,800,000
Trading account securities		4,300,000		1,700,000
Settlement of contingent consideration	55,300,000
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
Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Number of available for sale securities portfolio		843		831
Available for sale securities portfolio, value		24,600,000,000		26,400,000,000
Loans | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial Instrument, fair value		26,600,000	38,100,000
Other Real Estate Owned | Level 3
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Financial Instrument, fair value		1,500,000	1,600,000
External Pricing | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Number of available for sale securities portfolio		842		829
Available for sale securities portfolio, value		24,500,000,000		26,300,000,000
Trading account securities		4,300,000		1,700,000
Internal Pricing | Level 2
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Number of available for sale securities portfolio				2
Available for sale securities portfolio, value		$ 88,000,000		$ 57,400,000