Commitments and Contingencies - Company's Significant Commitments (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Commitments to extend credit
Home equity lines of credit	$ 5,631,680	$ 6,194,516
Commercial real estate loans to be sold	57,597	212,257
Other commercial real estate	5,949,933	5,161,273
Residential real estate loans to be sold	488,621	432,352
Other residential real estate	212,619	197,825
Commercial and other	11,802,850	11,080,856
Standby letters of credit	3,330,013	3,706,888
Commercial letters of credit	55,559	46,965
Financial guarantees and indemnification contracts	2,794,322	2,490,050
Commitments to sell real estate loans	$ 782,885	$ 1,237,294