Interest-Bearing Time Deposits in Banks and Securities - Amortized Cost and Estimated Fair Value of Debt Securities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012
Schedule Of Carrying Values And Estimated Fair Values Of Debt Instruments [Line Items]
Available-for-Sale Amortized Cost Basis, Due within one year	$ 156,237
Available-for-Sale Amortized Cost Basis, Due after one year through five years	494,482
Available-for-Sale Amortized Cost Basis, Due after five years through ten years	467,185
Available-for-Sale Amortized Cost Basis, Due after ten years	11,080
Available-for-Sale Amortized Cost Basis, Mortgage-backed securities	598,253
Available-for-Sale Amortized Cost Basis, Total	1,727,237
Available-for-Sale Estimated Fair Value, Due within one year	158,605
Available-for-Sale Estimated Fair Value, Due after one year through five years	514,761
Available-for-Sale Estimated Fair Value, Due after five years through ten years	509,127
Available-for-Sale Estimated Fair Value, Due after ten years	13,958
Available-for-Sale Estimated Fair Value, Mortgage-backed securities	622,584
Available-for-Sale Estimated Fair Value, Total	1,819,035
Held-to-Maturity Amortized Cost Basis, Due within one year	655
Held-to-Maturity Amortized Cost Basis, Due after one year through five years	80
Held-to-Maturity Amortized Cost Basis, Due after five year through ten years
Held-to-Maturity Amortized Cost Basis, Due after ten years
Held-to-Maturity Amortized Cost Basis, Mortgage-backed securities	326
Held-to-Maturity Amortized Cost Basis, Total	1,061
Held-to-Maturity Estimated Fair Value, Due within one year	658
Held-to-Maturity Estimated Fair Value, Due after one year through five years	84
Held-to-Maturity Estimated Fair Value, Due after five year through ten years
Held-to-Maturity Estimated Fair Value, Due after ten years
Held-to-Maturity Estimated Fair Value, Mortgage-backed securities	338
Held-to-Maturity Estimated Fair Value, Total	$ 1,080