Amortized Cost and Approximate Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Schedule of Available for Sale and Held-to-Maturity Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains	$ 18	$ 4	$ 243	$ 12
Available-for-sale Securities, Gross Realized Losses	1	16	155	16
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,501,036		1,501,036		1,971,757
Available-for-sale Securities, Gross Unrealized Gain1	14,519		14,519		13,346
Available-for-sale Securities, Gross Unrealized Loss1	(8,559)		(8,559)		(37,397)
Available-for-Sale, Estimated Fair Value	1,506,996		1,506,996		1,947,706
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	586,989		586,989		203,837
Held-to-maturity Securities, Gross Unrealized Gain1	6,293		6,293		4,146
Held-to-maturity Securities, Gross Unrealized Loss1	(600)		(600)		(2,057)
Held-to-Maturity, Estimated Fair Value	592,682		592,682		205,926
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	722,034		722,034		774,055
Available-for-sale Securities, Gross Unrealized Gain1	1,707		1,707		1,432
Available-for-sale Securities, Gross Unrealized Loss1	(5,174)		(5,174)		(12,249)
Available-for-Sale, Estimated Fair Value	718,567		718,567		763,238
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	778,641		778,641		1,197,295
Available-for-sale Securities, Gross Unrealized Gain1	12,806		12,806		11,905
Available-for-sale Securities, Gross Unrealized Loss1	(3,383)		(3,383)		(25,147)
Available-for-Sale, Estimated Fair Value	788,064		788,064		1,184,053
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	389,950		389,950
Held-to-maturity Securities, Gross Unrealized Gain1	18		18
Held-to-maturity Securities, Gross Unrealized Loss1	0		0
Held-to-Maturity, Estimated Fair Value	389,968		389,968
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	361		361		407
Available-for-sale Securities, Gross Unrealized Gain1	6		6		9
Available-for-sale Securities, Gross Unrealized Loss1	(2)		(2)		(1)
Available-for-Sale, Estimated Fair Value	365		365		415
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	179,131		179,131		185,818
Held-to-maturity Securities, Gross Unrealized Gain1	6,159		6,159		4,043
Held-to-maturity Securities, Gross Unrealized Loss1	(600)		(600)		(2,049)
Held-to-Maturity, Estimated Fair Value	184,690		184,690		187,812
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	17,908		17,908		18,019
Held-to-maturity Securities, Gross Unrealized Gain1	116		116		103
Held-to-maturity Securities, Gross Unrealized Loss1	0		0		(8)
Held-to-Maturity, Estimated Fair Value	$ 18,024		$ 18,024		$ 18,114