Fair Value - Schedule of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	$ 2,853,539	$ 2,048,442
Derivative assets	21,254	20,093
Total recurring fair value measurements-assets	2,874,793	2,068,535
Derivative liabilities	22,075	21,100
Total recurring fair value measurements-liabilities	22,075	21,100
U.S. Treasury and government agency securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	157	18,265
Municipal obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	55,913	50,165
Corporate debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	3,750	2,250
Mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	2,594,725	1,774,406
Collateralized mortgage obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	193,623	198,077
Equity securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	5,371	5,279
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	9,278	25,794
Total recurring fair value measurements-assets	9,278	25,794
Level 1 [Member] | U.S. Treasury and government agency securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	157	18,265
Level 1 [Member] | Corporate debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	3,750	2,250
Level 1 [Member] | Equity securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	5,371	5,279
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	2,844,261	2,022,648
Derivative assets	21,254	20,093
Total recurring fair value measurements-assets	2,865,515	2,042,741
Derivative liabilities	22,075	21,100
Total recurring fair value measurements-liabilities	22,075	21,100
Level 2 [Member] | Municipal obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	55,913	50,165
Level 2 [Member] | Mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	2,594,725	1,774,406
Level 2 [Member] | Collateralized mortgage obligations [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	$ 193,623	$ 198,077