Commitments and Contingencies - Company's Significant Commitments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Commitments to extend credit
Home equity lines of credit	$ 5,465,893	$ 5,482,622
Commercial real estate loans to be sold	153,900	194,763
Other commercial real estate	6,549,168	6,050,569
Residential real estate loans to be sold	307,955	347,113
Other residential real estate	267,948	201,426
Commercial and other	14,608,286	12,733,815
Standby letters of credit	2,385,904	2,497,844
Commercial letters of credit	42,030	46,739
Financial guarantees and indemnification contracts	3,416,625	3,434,381
Commitments to sell real estate loans	$ 994,879	$ 812,217