Commitments and Contingencies - Commitments and Contingent Liabilities Outstanding (Detail) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Commitments to extend credit
Home equity lines of credit	$ 5,677,025	$ 5,563,854
Commercial real estate loans to be sold	445,249	363,735
Other commercial real estate	6,077,907	7,237,367
Residential real estate loans to be sold	1,022,844	1,026,118
Other residential real estate	942,654	665,259
Commercial and other	21,003,234	19,427,886
Standby letters of credit	2,269,907	2,241,417
Commercial letters of credit	30,893	27,332
Financial guarantees and indemnification contracts	4,199,215	4,220,531
Commitments to sell real estate loans	$ 2,017,837	$ 2,108,823