Amortized Cost and Approximate Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Schedule of Available for Sale and Held-to-Maturity Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains	$ 30	$ 66	$ 42	$ 298
Available-for-sale Securities, Gross Realized Losses	0	0	(16)	(3)
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,950,473		1,950,473		1,966,417
Available-for-sale Securities, Gross Unrealized Gain1	16,421		16,421		30,531
Available-for-sale Securities, Gross Unrealized Loss1	(27,088)		(27,088)		(1,690)
Available-for-Sale, Estimated Fair Value	1,939,806		1,939,806		1,995,258
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	205,277		205,277		208,223
Held-to-maturity Securities, Gross Unrealized Gain1	5,085		5,085		10,899
Held-to-maturity Securities, Gross Unrealized Loss1	(1,880)		(1,880)		(189)
Held-to-Maturity, Estimated Fair Value	208,482		208,482		218,933
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	792,761		792,761		751,501
Available-for-sale Securities, Gross Unrealized Gain1	1,576		1,576		3,518
Available-for-sale Securities, Gross Unrealized Loss1	(9,862)		(9,862)		(163)
Available-for-Sale, Estimated Fair Value	784,475		784,475		754,856
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,157,281		1,157,281		1,214,377
Available-for-sale Securities, Gross Unrealized Gain1	14,836		14,836		27,000
Available-for-sale Securities, Gross Unrealized Loss1	(17,223)		(17,223)		(1,526)
Available-for-Sale, Estimated Fair Value	1,154,894		1,154,894		1,239,851
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	431		431		539
Available-for-sale Securities, Gross Unrealized Gain1	9		9		13
Available-for-sale Securities, Gross Unrealized Loss1	(3)		(3)		(1)
Available-for-Sale, Estimated Fair Value	437		437		551
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	187,202		187,202		192,875
Held-to-maturity Securities, Gross Unrealized Gain1	5,011		5,011		10,835
Held-to-maturity Securities, Gross Unrealized Loss1	(1,861)		(1,861)		(176)
Held-to-Maturity, Estimated Fair Value	190,352		190,352		203,534
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	18,075		18,075		14,975
Held-to-maturity Securities, Gross Unrealized Gain1	74		74		64
Held-to-maturity Securities, Gross Unrealized Loss1	(19)		(19)		(13)
Held-to-Maturity, Estimated Fair Value	18,130		18,130		15,026
Other securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost					373
Held-to-maturity Securities, Gross Unrealized Gain1					0
Held-to-maturity Securities, Gross Unrealized Loss1					0
Held-to-Maturity, Estimated Fair Value					$ 373