Commitments and Contingencies - Company's Significant Commitments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Commitments to extend credit
Home equity lines of credit	$ 5,482,622	$ 5,499,609
Commercial real estate loans to be sold	194,763	70,100
Other commercial real estate	6,050,569	6,451,709
Residential real estate loans to be sold	347,113	478,950
Other residential real estate	201,426	232,721
Commercial and other	12,733,815	12,298,473
Standby letters of credit	2,497,844	2,987,091
Commercial letters of credit	46,739	44,723
Financial guarantees and indemnification contracts	3,434,381	3,043,580
Commitments to sell real estate loans	$ 812,217	$ 1,489,237