Commitments and Contingencies - Company's Significant Commitments (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Commitments to extend credit
Home equity lines of credit	$ 5,535,704	$ 6,194,516
Commercial real estate loans to be sold	89,374	212,257
Other commercial real estate and construction	5,356,256	4,834,699
Residential real estate loans to be sold	587,206	432,352
Other residential real estate	674,338	524,399
Commercial and other	12,144,962	11,080,856
Standby letters of credit	3,441,337	3,706,888
Commercial letters of credit	44,082	46,965
Financial guarantees and indemnification contracts	2,922,743	2,490,050
Commitments to sell real estate loans	$ 977,822	$ 1,237,294