Amortized Cost and Approximate Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Schedule of Available for Sale and Held-to-Maturity Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains	$ 0	$ 30	$ 243	$ 42
Available-for-sale Securities, Gross Realized Losses	(8)	0	(163)	(16)
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,584,603		1,584,603		1,971,757
Available-for-sale Securities, Gross Unrealized Gain1	12,350		12,350		13,346
Available-for-sale Securities, Gross Unrealized Loss1	(10,450)		(10,450)		(37,397)
Available-for-Sale, Estimated Fair Value	1,586,503		1,586,503		1,947,706
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	583,271		583,271		203,837
Held-to-maturity Securities, Gross Unrealized Gain1	7,423		7,423		4,146
Held-to-maturity Securities, Gross Unrealized Loss1	(2,662)		(2,662)		(2,057)
Held-to-Maturity, Estimated Fair Value	588,032		588,032		205,926
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	733,144		733,144		774,055
Available-for-sale Securities, Gross Unrealized Gain1	1,193		1,193		1,432
Available-for-sale Securities, Gross Unrealized Loss1	(5,840)		(5,840)		(12,249)
Available-for-Sale, Estimated Fair Value	728,497		728,497		763,238
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	851,119		851,119		1,197,295
Available-for-sale Securities, Gross Unrealized Gain1	11,152		11,152		11,905
Available-for-sale Securities, Gross Unrealized Loss1	(4,609)		(4,609)		(25,147)
Available-for-Sale, Estimated Fair Value	857,662		857,662		1,184,053
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	370,933		370,933
Held-to-maturity Securities, Gross Unrealized Gain1	1,081		1,081
Held-to-maturity Securities, Gross Unrealized Loss1	(2,208)		(2,208)
Held-to-Maturity, Estimated Fair Value	369,806		369,806
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	340		340		407
Available-for-sale Securities, Gross Unrealized Gain1	5		5		9
Available-for-sale Securities, Gross Unrealized Loss1	(1)		(1)		(1)
Available-for-Sale, Estimated Fair Value	344		344		415
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	189,919		189,919		185,818
Held-to-maturity Securities, Gross Unrealized Gain1	6,260		6,260		4,043
Held-to-maturity Securities, Gross Unrealized Loss1	(454)		(454)		(2,049)
Held-to-Maturity, Estimated Fair Value	195,725		195,725		187,812
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	21,860		21,860		18,019
Held-to-maturity Securities, Gross Unrealized Gain1	82		82		103
Held-to-maturity Securities, Gross Unrealized Loss1	0		0		(8)
Held-to-Maturity, Estimated Fair Value	$ 21,942		$ 21,942		$ 18,114