Total interest bearing deposits (Detail) - Entity [Domain] - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Deposit Liabilities [Line Items]
Deposits Savings Deposits	$ 6,991,645	$ 6,737,370
Deposits Negotiable Order Of Withdrawal N O W Money Market Account And Other Demand Deposits	5,610,806	4,811,972
Total savings, NOW, money market and other interest bearing demand deposits	12,602,451	11,549,342
Time Deposits Abstract
Time Deposits Less Than 100000	4,324,540	4,211,180
Time Deposits 100000 Or More	4,517,717	3,263,265
Total certificates of deposit	8,842,257	7,474,445
Total interest bearing deposits	$ 21,444,708	$ 19,023,787