Commitments and Contingencies - Company's Significant Commitments (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Commitments to extend credit
Home equity lines of credit	$ 5,518,820	$ 5,482,622
Commercial real estate loans to be sold	288,883	194,763
Other commercial real estate	5,788,080	6,050,569
Residential real estate loans to be sold	343,400	347,113
Other residential real estate	223,589	201,426
Commercial and other	12,951,589	12,733,815
Standby letters of credit	2,477,474	2,497,844
Commercial letters of credit	56,245	46,739
Financial guarantees and indemnification contracts	3,449,447	3,434,381
Commitments to sell real estate loans	$ 977,257	$ 812,217