Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Schedule of Available for Sale and Held-to-Maturity Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains	$ 46	$ 18	$ 52	$ 243
Available-for-sale Securities, Gross Realized Losses	0	$ 1	0	$ 155
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,587,953		1,587,953		$ 1,708,494
Available-for-sale Securities, Gross Unrealized Gain1	11,559		11,559		12,426
Available-for-sale Securities, Gross Unrealized Loss1	(6,769)		(6,769)		(8,996)
Available-for-Sale, Estimated Fair Value	1,592,743		1,592,743		1,711,924
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	546,690		546,690		575,186
Held-to-maturity Securities, Gross Unrealized Gain1	10,649		10,649		11,566
Held-to-maturity Securities, Gross Unrealized Loss1	(4,016)		(4,016)		(2,219)
Held-to-Maturity, Estimated Fair Value	553,323		553,323		584,533
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	622,237		622,237		725,408
Available-for-sale Securities, Gross Unrealized Gain1	1,087		1,087		895
Available-for-sale Securities, Gross Unrealized Loss1	(2,304)		(2,304)		(5,370)
Available-for-Sale, Estimated Fair Value	621,020		621,020		720,933
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	965,430		965,430		982,764
Available-for-sale Securities, Gross Unrealized Gain1	10,468		10,468		11,526
Available-for-sale Securities, Gross Unrealized Loss1	(4,463)		(4,463)		(3,624)
Available-for-Sale, Estimated Fair Value	971,435		971,435		990,666
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	314,326		314,326		353,176
Held-to-maturity Securities, Gross Unrealized Gain1	5,945		5,945		5,563
Held-to-maturity Securities, Gross Unrealized Loss1	(3,288)		(3,288)		(1,758)
Held-to-Maturity, Estimated Fair Value	316,983		316,983		356,981
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	286		286		322
Available-for-sale Securities, Gross Unrealized Gain1	4		4		5
Available-for-sale Securities, Gross Unrealized Loss1	(2)		(2)		(2)
Available-for-Sale, Estimated Fair Value	288		288		325
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	181,642		181,642		188,941
Held-to-maturity Securities, Gross Unrealized Gain1	4,644		4,644		5,949
Held-to-maturity Securities, Gross Unrealized Loss1	(524)		(524)		(386)
Held-to-Maturity, Estimated Fair Value	185,762		185,762		194,504
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	50,310		50,310		32,565
Held-to-maturity Securities, Gross Unrealized Gain1	59		59		54
Held-to-maturity Securities, Gross Unrealized Loss1	(204)		(204)		(75)
Held-to-Maturity, Estimated Fair Value	50,165		50,165		32,544
Other Investments
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	412		412		504
Held-to-maturity Securities, Gross Unrealized Gain1	1		1		0
Held-to-maturity Securities, Gross Unrealized Loss1	0		0		0
Held-to-Maturity, Estimated Fair Value	$ 413		$ 413		$ 504