Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Summary Information Related to Commitments to Extend Credit (Excluding Letters of Credit) (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Disclosure Off Balance Sheet Arrangements Guarantees And Other Commitments Additional Information [Abstract]
Fixed interest rate commitments	$ 1,393,396	$ 1,591,408
Variable interest rate commitments	13,302,099	11,860,039
Total loan commitments available for funding	14,695,495	13,451,447
Commercial and standby letters of credit	1,391,812	1,254,338
Total unfunded credit commitments	16,087,307	14,705,785
Commitments unavailable for funding	2,141,183	1,868,489
Maximum lending limits for accounts receivable factoring arrangements	1,084,623	1,044,548
Reserve for unfunded credit commitments	$ 36,631	$ 36,419