Commitments and Contingencies - Company's Significant Commitments (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Commitments to extend credit
Home equity lines of credit	$ 5,604,610	$ 5,631,680
Commercial real estate loans to be sold	184,198	57,597
Other commercial real estate	5,727,946	5,949,933
Residential real estate loans to be sold	520,694	488,621
Other residential real estate	281,735	212,619
Commercial and other	12,077,612	11,802,850
Standby letters of credit	3,330,241	3,330,013
Commercial letters of credit	45,798	55,559
Financial guarantees and indemnification contracts	2,773,590	2,794,322
Commitments to sell real estate loans	$ 958,337	$ 782,885