Off-Balance Sheet Arrangements, Guarantees and Other Commitments Summary Information Related to Commitments to Extend Credit (Excluding Letters of Credit) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Disclosure Summary Information Related To Commitments To Extend Credit [Abstract]
Fixed interest rate commitments	$ 1,478,157	$ 1,475,179
Variable interest rate commitments	14,034,169	13,572,161
Total loan commitments available for funding	15,512,326	15,047,340
Commercial and standby letters of credit	1,950,211	1,695,856
Total unfunded credit commitments	17,462,537	16,743,196
Commitments unavailable for funding	2,117,057	1,719,524
Reserve For Unfunded Credit Commitments	$ 51,770	$ 45,265	$ 34,415	$ 36,419