Commitments and Contingencies - Company's Significant Commitments (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Commitments to extend credit
Home equity lines of credit	$ 5,474,798	$ 5,482,622
Commercial real estate loans to be sold	323,750	194,763
Other commercial real estate	5,855,075	6,050,569
Residential real estate loans to be sold	361,034	347,113
Other residential real estate	240,020	201,426
Commercial and other	13,247,615	12,733,815
Standby letters of credit	2,410,569	2,497,844
Commercial letters of credit	55,655	46,739
Financial guarantees and indemnification contracts	3,458,625	3,434,381
Commitments to sell real estate loans	$ 1,725,093	$ 812,217