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, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Disclosure Off Balance Sheet Arrangements Guarantees And Other Commitments Additional Information [Abstract]
Fixed interest rate commitments	$ 1,768,904		$ 1,478,157
Variable interest rate commitments	14,698,357		14,034,169
Total loan commitments available for funding	16,467,261		15,512,326
Commercial and standby letters of credit	2,072,253		1,950,211
Total unfunded credit commitments	18,539,514		17,462,537
Commitments unavailable for funding	2,458,846		2,117,057
Reserve for unfunded commitments	$ 51,808	$ 54,104	$ 51,770	$ 48,172	$ 47,000	$ 45,265