Amortized Cost and Approximate Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2012	Dec. 31, 2012
Schedule of Available for Sale and Held-to-Maturity Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains	$ 4	$ 201	$ 232
Available-for-sale Securities, Gross Realized Losses	16	3	3
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,935,647			1,966,417
Available-for-sale Securities, Gross Unrealized Gain1	16,950			30,531
Available-for-sale Securities, Gross Unrealized Loss1	(29,381)			(1,690)
Available-for-Sale, Estimated Fair Value	1,923,216			1,995,258
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	215,323			208,223
Held-to-maturity Securities, Gross Unrealized Gain1	5,794			10,899
Held-to-maturity Securities, Gross Unrealized Loss1	(2,089)			(189)
Held-to-Maturity, Estimated Fair Value	219,028			218,933
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	744,215			751,501
Available-for-sale Securities, Gross Unrealized Gain1	1,158			3,518
Available-for-sale Securities, Gross Unrealized Loss1	(11,559)			(163)
Available-for-Sale, Estimated Fair Value	733,814			754,856
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,190,975			1,214,377
Available-for-sale Securities, Gross Unrealized Gain1	15,783			27,000
Available-for-sale Securities, Gross Unrealized Loss1	(17,819)			(1,526)
Available-for-Sale, Estimated Fair Value	1,188,939			1,239,851
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	457			539
Available-for-sale Securities, Gross Unrealized Gain1	9			13
Available-for-sale Securities, Gross Unrealized Loss1	(3)			(1)
Available-for-Sale, Estimated Fair Value	463			551
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	197,193			192,875
Held-to-maturity Securities, Gross Unrealized Gain1	5,730			10,835
Held-to-maturity Securities, Gross Unrealized Loss1	(1,990)			(176)
Held-to-Maturity, Estimated Fair Value	200,933			203,534
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	18,130			14,975
Held-to-maturity Securities, Gross Unrealized Gain1	64			64
Held-to-maturity Securities, Gross Unrealized Loss1	(99)			(13)
Held-to-Maturity, Estimated Fair Value	18,095			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