Commitments and Contingencies - Commitments and Contingent Liabilities Outstanding (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Commitments to extend credit
Home equity lines of credit	$ 5,557,722	$ 5,442,160
Commercial real estate loans to be sold	216,353	164,076
Other commercial real estate	8,031,403	9,029,608
Residential real estate loans to be sold	1,134,097	423,056
Other residential real estate	712,964	448,375
Commercial and other	19,921,166	16,170,731
Standby letters of credit	2,354,799	2,441,432
Commercial letters of credit	32,422	41,059
Financial guarantees and indemnification contracts	4,250,654	4,108,572
Commitments to sell real estate loans	$ 1,895,436	$ 906,037