Fair Value of Financial Instruments - Schedule of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	$ 2,048,442	$ 4,496,900
Derivative assets	20,093
Total recurring fair value measurements - assets	2,068,535	4,511,852
Derivative liabilities	21,100
Total recurring fair value measurements - liabilities	21,100	15,643
U.S. Treasury and government agency securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	18,265	250,067
Municipal Obligation [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	50,165	309,665
Corporate debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	2,250	4,494
Mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	1,774,406	2,480,345
CMOs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	198,077	1,446,076
Equity securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	5,279	6,253
Interest rate contracts [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivative assets		14,952
Derivative liabilities		15,643
Level 1 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	25,794	260,814
Total recurring fair value measurements - assets	25,794	260,814
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	18,265	250,067
Level 1 [Member] | Corporate debt securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	2,250	4,494
Level 1 [Member] | Equity securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	5,279	6,253
Level 2 [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	2,022,648	4,236,086
Derivative assets	20,093	14,952
Total recurring fair value measurements - assets	2,042,741	4,251,038
Derivative liabilities	21,100	15,643
Total recurring fair value measurements - liabilities	21,100	15,643
Level 2 [Member] | Municipal Obligation [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	50,165	309,665
Level 2 [Member] | Mortgage-backed securities [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	1,774,406	2,480,345
Level 2 [Member] | CMOs [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Available for sale debt securities	198,077	1,446,076
Level 2 [Member] | Interest rate contracts [Member]
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]
Derivative assets		14,952
Derivative liabilities		$ 15,643