Securities - Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, amortized cost	$ 2,804,411	$ 1,986,882
Securities available for sale, Gross Unrealized Gains	54,886	61,599
Securities available for sale, Gross Unrealized Losses	5,758	39
Securities Available for Sale, Fair Value	2,853,539	2,048,442
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, amortized cost	150	150
Securities available for sale, Gross Unrealized Gains	7	8
Securities Available for Sale, Fair Value	157	158
U.S. government agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, amortized cost		18,096
Securities available for sale, Gross Unrealized Gains		11
Securities Available for Sale, Fair Value		18,107
Municipal Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, amortized cost	55,405	49,608
Securities available for sale, Gross Unrealized Gains	554	571
Securities available for sale, Gross Unrealized Losses	46	14
Securities Available for Sale, Fair Value	55,913	50,165
Mortgage-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, amortized cost	2,547,382	1,715,524
Securities available for sale, Gross Unrealized Gains	52,999	58,903
Securities available for sale, Gross Unrealized Losses	5,656	21
Securities Available for Sale, Fair Value	2,594,725	1,774,406
Collateralized mortgage obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, amortized cost	193,153	196,723
Securities available for sale, Gross Unrealized Gains	525	1,354
Securities available for sale, Gross Unrealized Losses	55
Securities Available for Sale, Fair Value	193,623	198,077
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, amortized cost	3,750	2,250
Securities Available for Sale, Fair Value	3,750	2,250
Other equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Securities available for sale, amortized cost	4,571	4,531
Securities available for sale, Gross Unrealized Gains	801	752
Securities available for sale, Gross Unrealized Losses	1	4
Securities Available for Sale, Fair Value	$ 5,371	$ 5,279

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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+ Details

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References+ Details

- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References+ Details