Fair Value Information - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Carrying Value [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Total assets	$ 2,412,031	$ 2,473,353
Total liabilities	21,358	23,966
Carrying Value [Member] | Government Agency [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investment securities - AFS	390,638	359,300
Carrying Value [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investment securities - AFS	892,637	887,598
Carrying Value [Member] | CMO's / REMIC's - residential [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investment securities - AFS	477,577	571,960
Carrying Value [Member] | Municipal bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investment securities - AFS	624,799	625,429
Carrying Value [Member] | Other Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investment securities - AFS	5,022	5,100
Carrying Value [Member] | Investment securities - AFS [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investment securities - AFS	2,390,673	2,449,387
Carrying Value [Member] | Interest rate swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Interest rate swaps	21,358	23,966
Interest rate swaps	21,358	23,966
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Interest rate swaps	21,358	23,966
Total assets	2,412,031	2,473,353
Interest rate swaps	21,358	23,966
Total liabilities	21,358	23,966
Level 2 [Member] | Government Agency [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investment securities - AFS	390,638	359,300
Level 2 [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investment securities - AFS	892,637	887,598
Level 2 [Member] | CMO's / REMIC's - residential [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investment securities - AFS	477,577	571,960
Level 2 [Member] | Municipal bonds [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investment securities - AFS	624,799	625,429
Level 2 [Member] | Other Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investment securities - AFS	5,022	5,100
Level 2 [Member] | Investment securities - AFS [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Investment securities - AFS	2,390,673	2,449,387
Level 2 [Member] | Interest rate swaps [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Interest rate swaps	21,358	23,966
Interest rate swaps	$ 21,358	$ 23,966