Commitments and Contingencies - Company's Significant Commitments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Commitments to extend credit
Home equity lines of credit	$ 6,219,783	$ 6,194,516
Commercial real estate loans to be sold	346,664	212,257
Other commercial real estate and construction	5,161,878	4,834,699
Residential real estate loans to be sold	661,132	432,352
Other residential real estate	581,384	524,399
Commercial and other	11,493,613	11,080,856
Standby letters of credit	3,648,095	3,706,888
Commercial letters of credit	42,291	46,965
Financial guarantees and indemnification contracts	2,535,609	2,490,050
Commitments to sell real estate loans	$ 1,322,998	$ 1,237,294