Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Available-for-Sale:
Available-for-Sale, Amortized Cost	$ 1,971,757	$ 1,966,417
Available-for-sale Securities, Gross Unrealized Gain1	13,346	30,531
Available-for-sale Securities, Gross Unrealized Loss1	(37,397)	(1,690)
Available-for-Sale, Estimated Fair Value	1,947,706	1,995,258
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	203,837	208,223
Held-to-maturity Securities, Gross Unrealized Gain1	4,146	10,899
Held-to-maturity Securities, Gross Unrealized Loss1	(2,057)	(189)
Held-to-maturity estimated fair value, total	205,926	218,933
Gross realized gains	49	351	1,544
Gross realized losses	48	3	0
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	774,055	751,501
Available-for-sale Securities, Gross Unrealized Gain1	1,432	3,518
Available-for-sale Securities, Gross Unrealized Loss1	(12,249)	(163)
Available-for-Sale, Estimated Fair Value	763,238	754,856
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,197,295	1,214,377
Available-for-sale Securities, Gross Unrealized Gain1	11,905	27,000
Available-for-sale Securities, Gross Unrealized Loss1	(25,147)	(1,526)
Available-for-Sale, Estimated Fair Value	1,184,053	1,239,851
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	407	539
Available-for-sale Securities, Gross Unrealized Gain1	9	13
Available-for-sale Securities, Gross Unrealized Loss1	(1)	(1)
Available-for-Sale, Estimated Fair Value	415	551
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	185,818	192,875
Held-to-maturity Securities, Gross Unrealized Gain1	4,043	10,835
Held-to-maturity Securities, Gross Unrealized Loss1	(2,049)	(176)
Held-to-maturity estimated fair value, total	187,812	203,534
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	18,019	14,975
Held-to-maturity Securities, Gross Unrealized Gain1	103	64
Held-to-maturity Securities, Gross Unrealized Loss1	(8)	(13)
Held-to-maturity estimated fair value, total	18,114	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, total		$ 373