Commitments and Contingencies - Company's Significant Commitments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Commitments to extend credit
Home equity lines of credit	$ 5,529,111	$ 5,484,197
Commercial real estate loans to be sold	199,722	229,401
Other commercial real estate	7,331,360	7,556,722
Residential real estate loans to be sold	314,402	245,211
Other residential real estate	277,708	219,351
Commercial and other	14,900,142	14,363,803
Standby letters of credit	2,246,864	2,326,991
Commercial letters of credit	59,464	55,808
Financial guarantees and indemnification contracts	3,703,612	3,529,136
Commitments to sell real estate loans	$ 746,307	$ 940,692