Amortized Cost and Approximate Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Schedule of Available for Sale and Held-to-Maturity Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains	$ 8	$ 31
Available-for-sale Securities, Gross Realized Losses	0	0
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,987,964		1,966,417
Available-for-Sale, Gross Unrealized Gains	27,049		30,531
Available-for-Sale, Gross Unrealized Losses	(3,844)		(1,690)
Available-for-Sale, Estimated Fair Value	2,011,169		1,995,258
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	210,426		208,223
Held-to-Maturity, Gross Unrealized Gains	9,977		10,899
Held-to-Maturity, Gross Unrealized Losses	(288)		(189)
Held-to-Maturity, Estimated Fair Value	220,115		218,933
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	777,044		751,501
Available-for-Sale, Gross Unrealized Gains	3,220		3,518
Available-for-Sale, Gross Unrealized Losses	(342)		(163)
Available-for-Sale, Estimated Fair Value	779,922		754,856
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,210,425		1,214,377
Available-for-Sale, Gross Unrealized Gains	23,814		27,000
Available-for-Sale, Gross Unrealized Losses	(3,501)		(1,526)
Available-for-Sale, Estimated Fair Value	1,230,738		1,239,851
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	495		539
Available-for-Sale, Gross Unrealized Gains	15		13
Available-for-Sale, Gross Unrealized Losses	(1)		(1)
Available-for-Sale, Estimated Fair Value	509		551
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	192,241		192,875
Held-to-Maturity, Gross Unrealized Gains	9,881		10,835
Held-to-Maturity, Gross Unrealized Losses	(282)		(176)
Held-to-Maturity, Estimated Fair Value	201,840		203,534
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	18,185		14,975
Held-to-Maturity, Gross Unrealized Gains	96		64
Held-to-Maturity, Gross Unrealized Losses	(6)		(13)
Held-to-Maturity, Estimated Fair Value	18,275		15,026
Other securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost			373
Held-to-Maturity, Gross Unrealized Gains			0
Held-to-Maturity, Gross Unrealized Losses			0
Held-to-Maturity, Estimated Fair Value			$ 373

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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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.

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