Commitments and Contingencies - Company's Significant Commitments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Commitments to extend credit
Home equity lines of credit	$ 5,484,197	$ 5,482,622
Commercial real estate loans to be sold	229,401	194,763
Other commercial real estate	7,556,722	6,050,569
Residential real estate loans to be sold	245,211	347,113
Other residential real estate	219,351	201,426
Commercial and other	14,363,803	12,733,815
Standby letters of credit	2,326,991	2,497,844
Commercial letters of credit	55,808	46,739
Financial guarantees and indemnification contracts	3,529,136	3,434,381
Commitments to sell real estate loans	$ 940,692	$ 812,217