Amortized Cost and Approximate Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Schedule of Available for Sale and Held-to-Maturity Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains	$ 225	$ 8
Available-for-sale Securities, Gross Realized Losses	154	0
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,903,413		1,971,757
Available-for-sale Securities, Gross Unrealized Gain1	14,886		13,346
Available-for-sale Securities, Gross Unrealized Loss1	(24,647)		(37,397)
Available-for-Sale, Estimated Fair Value	1,893,652		1,947,706
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	201,436		203,837
Held-to-maturity Securities, Gross Unrealized Gain1	5,216		4,146
Held-to-maturity Securities, Gross Unrealized Loss1	(1,209)		(2,057)
Held-to-Maturity, Estimated Fair Value	205,443		205,926
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	712,895		774,055
Available-for-sale Securities, Gross Unrealized Gain1	1,563		1,432
Available-for-sale Securities, Gross Unrealized Loss1	(7,866)		(12,249)
Available-for-Sale, Estimated Fair Value	706,592		763,238
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,190,134		1,197,295
Available-for-sale Securities, Gross Unrealized Gain1	13,316		11,905
Available-for-sale Securities, Gross Unrealized Loss1	(16,779)		(25,147)
Available-for-Sale, Estimated Fair Value	1,186,671		1,184,053
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	384		407
Available-for-sale Securities, Gross Unrealized Gain1	7		9
Available-for-sale Securities, Gross Unrealized Loss1	(2)		(1)
Available-for-Sale, Estimated Fair Value	389		415
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	183,472		185,818
Held-to-maturity Securities, Gross Unrealized Gain1	5,101		4,043
Held-to-maturity Securities, Gross Unrealized Loss1	(1,209)		(2,049)
Held-to-Maturity, Estimated Fair Value	187,364		187,812
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	17,964		18,019
Held-to-maturity Securities, Gross Unrealized Gain1	115		103
Held-to-maturity Securities, Gross Unrealized Loss1	0		(8)
Held-to-Maturity, Estimated Fair Value	$ 18,079		$ 18,114