Commitments and Contingencies - Commitments and Contingent Liabilities Outstanding (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Commitments to extend credit
Home equity lines of credit	$ 5,687,595	$ 5,563,854
Commercial real estate loans to be sold	433,529	363,735
Other commercial real estate	5,747,541	7,237,367
Residential real estate loans to be sold	750,820	1,026,118
Other residential real estate	840,039	665,259
Commercial and other	21,685,356	19,427,886
Standby letters of credit	2,236,321	2,241,417
Commercial letters of credit	34,495	27,332
Financial guarantees and indemnification contracts	4,133,648	4,220,531
Commitments to sell real estate loans	$ 1,788,113	$ 2,108,823