Interest-bearing Time Deposits in Banks and Securities - Amortized Cost and Estimated Fair Value of Debt Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013
Investments Debt And Equity Securities [Abstract]
Available-for-Sale, Amortized Cost Basis, Due within one year	$ 142,205
Available-for-Sale, Amortized Cost Basis, Due after one year through five years	509,641
Available-for-Sale, Amortized Cost Basis, Due after five years through ten years	495,789
Available-for-Sale, Amortized Cost Basis, Due after ten years	25,246
Available-for-Sale, Amortized Cost Basis, Mortgage-backed securities	699,809
Available-for-Sale, Amortized Cost Basis, Total	1,872,690
Available-for-Sale, Estimated Fair Value, Due within one year	144,366
Available-for-Sale, Estimated Fair Value, Due after one year through five years	532,461
Available-for-Sale, Estimated Fair Value, Due after five years through ten years	530,064
Available-for-Sale, Estimated Fair Value, Due after ten years	27,650
Available-for-Sale, Estimated Fair Value, Mortgage-backed securities	723,610
Available-for-Sale, Estimated Fair Value, Total	1,958,151
Held-to-Maturity, Amortized Cost Basis, Due within one year	595
Held-to-Maturity, Amortized Cost Basis, Due after one year through five years
Held-to-Maturity, Amortized Cost Basis, Due after five years through ten years
Held-to-Maturity, Amortized Cost Basis, Due after ten years
Held-to-Maturity, Amortized Cost Basis, Mortgage-backed securities	308
Held-to-Maturity, Amortized Cost Basis, Total	903
Held-to-Maturity, Estimated Fair Value, Due within one year	600
Held-to-Maturity, Estimated Fair Value, Due after one year through five years
Held-to-Maturity, Estimated Fair Value, Due after five years through ten years
Held-to-Maturity, Estimated Fair Value, Due after ten years
Held-to-Maturity, Estimated Fair Value, Mortgage-backed securities	320
Held-to-Maturity, Estimated Fair Value, Total	$ 920