Interest-bearing Time Deposits in Banks and Securities - Summary of Available-for-Sale and Held-to-Maturity Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	$ 1,872,690	$ 1,727,237
Securities available-for-sale, Gross Unrealized Holding Gains	87,492	92,693
Securities available-for-sale, Gross Unrealized Holding Losses	(2,031)	(895)
Securities available-for-sale, Estimated Fair Value	1,958,151	1,819,035	1,960,760
Securities held-to-maturity, Amortized Cost Basis	903	1,061	2,581
Securities held-to-maturity, Gross Unrealized Holding Gains	17	19
Securities held-to-maturity, Gross Unrealized Holding Losses
Securities held-to-maturity, Estimated Fair Value	920	1,080	2,612
U.S. Treasury securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	6,017	6,042
Securities available-for-sale, Gross Unrealized Holding Gains	23	48
Securities available-for-sale, Gross Unrealized Holding Losses
Securities available-for-sale, Estimated Fair Value	6,040	6,090
Obligations of U.S. government sponsored-enterprises and agencies [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	189,913	219,420
Securities available-for-sale, Gross Unrealized Holding Gains	3,451	4,060
Securities available-for-sale, Gross Unrealized Holding Losses
Securities available-for-sale, Estimated Fair Value	193,364	223,480
Obligations of states and political subdivisions [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	865,282	786,278
Securities available-for-sale, Gross Unrealized Holding Gains	53,191	57,541
Securities available-for-sale, Gross Unrealized Holding Losses	(558)	(129)
Securities available-for-sale, Estimated Fair Value	917,915	843,690
Securities held-to-maturity, Amortized Cost Basis	595	735
Securities held-to-maturity, Gross Unrealized Holding Gains	5	7
Securities held-to-maturity, Gross Unrealized Holding Losses
Securities held-to-maturity, Estimated Fair Value	600	742
Corporate bonds and other [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	111,669	117,244
Securities available-for-sale, Gross Unrealized Holding Gains	5,553	6,020
Securities available-for-sale, Gross Unrealized Holding Losses		(73)
Securities available-for-sale, Estimated Fair Value	117,222	123,191
Residential mortgage-backed securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	592,911	564,434
Securities available-for-sale, Gross Unrealized Holding Gains	23,866	23,285
Securities available-for-sale, Gross Unrealized Holding Losses	(213)	(443)
Securities available-for-sale, Estimated Fair Value	616,564	587,276
Securities held-to-maturity, Amortized Cost Basis	308	294
Securities held-to-maturity, Gross Unrealized Holding Gains	12	11
Securities held-to-maturity, Gross Unrealized Holding Losses
Securities held-to-maturity, Estimated Fair Value	320	305
Commercial mortgage-backed securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Securities available-for-sale, Amortized Cost Basis	106,898	33,819
Securities available-for-sale, Gross Unrealized Holding Gains	1,408	1,739
Securities available-for-sale, Gross Unrealized Holding Losses	(1,260)	(250)
Securities available-for-sale, Estimated Fair Value	107,046	35,308
Securities held-to-maturity, Amortized Cost Basis		32
Securities held-to-maturity, Gross Unrealized Holding Gains		1
Securities held-to-maturity, Gross Unrealized Holding Losses
Securities held-to-maturity, Estimated Fair Value		$ 33

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

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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

- Definition

For an unclassified balance sheet, this item represents investments in debt securities which are categorized as held-to-maturity. The held-to-maturity category is for those securities that the entity has the positive intent and ability to hold until maturity.

+ References

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-Publisher AICPA

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-Subparagraph (SX 210.9-03.6)

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+ Details

- Definition

This item represents the aggregate fair value of investments in debt securities as of the balance sheet date and which are categorized as held-to-maturity.

+ References+ Details

- Definition

As of the balance sheet date, the excess of fair value over carrying value (amortized cost) of a debt security categorized as held-to-maturity, exclusive of dividend or interest income recognized but not yet received and exclusive of any write-downs for other-than-temporary impairment. Such item represents the gross unrecognized holding gain.

+ References+ Details

- Definition

As of the balance sheet date, the excess of carrying value (amortized cost) over fair value of a debt security categorized as held-to-maturity, exclusive of dividend or interest income recognized but not yet received and exclusive of any write-downs for other-than-temporary impairment. Such item represents the gross unrecognized holding loss.

+ References+ Details