Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Summary Information Related to Commitments to Extend Credit (Excluding Letters of Credit) (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Disclosure Off Balance Sheet Arrangements Guarantees And Other Commitments Additional Information [Abstract]
Fixed interest rate commitments	$ 1,513,974	$ 1,478,157
Variable interest rate commitments	13,665,905	14,034,169
Total loan commitments available for funding	15,179,879	15,512,326
Commercial and standby letters of credit	1,990,923	1,950,211
Total unfunded credit commitments	17,170,802	17,462,537
Commitments unavailable for funding	2,602,796	2,117,057
Reserve For Unfunded Credit Commitments	$ 52,823	$ 51,770	$ 46,335	$ 45,265