Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Schedule of Available for Sale and Held-to-Maturity Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains	$ 23	$ 0	$ 75	$ 243
Available-for-sale Securities, Gross Realized Losses	0	$ 8	0	$ 163
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,529,188		1,529,188		$ 1,708,494
Available-for-sale Securities, Gross Unrealized Gain1	17,541		17,541		12,426
Available-for-sale Securities, Gross Unrealized Loss1	(1,636)		(1,636)		(8,996)
Available-for-Sale, Estimated Fair Value	1,545,093		1,545,093		1,711,924
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	522,543		522,543		575,186
Held-to-maturity Securities, Gross Unrealized Gain1	16,004		16,004		11,566
Held-to-maturity Securities, Gross Unrealized Loss1	(3,914)		(3,914)		(2,219)
Held-to-Maturity, Estimated Fair Value	534,633		534,633		584,533
US Treasury Securities [Member]
Available-for-Sale:
Available-for-Sale, Amortized Cost	3,913		3,913
Available-for-sale Securities, Gross Unrealized Gain1	28		28
Available-for-sale Securities, Gross Unrealized Loss1	0		0
Available-for-Sale, Estimated Fair Value	3,941		3,941
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	604,198		604,198		725,408
Available-for-sale Securities, Gross Unrealized Gain1	2,196		2,196		895
Available-for-sale Securities, Gross Unrealized Loss1	(350)		(350)		(5,370)
Available-for-Sale, Estimated Fair Value	606,044		606,044		720,933
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	917,025		917,025		982,764
Available-for-sale Securities, Gross Unrealized Gain1	15,314		15,314		11,526
Available-for-sale Securities, Gross Unrealized Loss1	(1,284)		(1,284)		(3,624)
Available-for-Sale, Estimated Fair Value	931,055		931,055		990,666
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	295,418		295,418		353,176
Held-to-maturity Securities, Gross Unrealized Gain1	10,346		10,346		5,563
Held-to-maturity Securities, Gross Unrealized Loss1	(3,665)		(3,665)		(1,758)
Held-to-Maturity, Estimated Fair Value	302,099		302,099		356,981
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	257		257		322
Available-for-sale Securities, Gross Unrealized Gain1	3		3		5
Available-for-sale Securities, Gross Unrealized Loss1	(2)		(2)		(2)
Available-for-Sale, Estimated Fair Value	258		258		325
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	176,569		176,569		188,941
Held-to-maturity Securities, Gross Unrealized Gain1	5,471		5,471		5,949
Held-to-maturity Securities, Gross Unrealized Loss1	(200)		(200)		(386)
Held-to-Maturity, Estimated Fair Value	181,840		181,840		194,504
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	50,178		50,178		32,565
Held-to-maturity Securities, Gross Unrealized Gain1	186		186		54
Held-to-maturity Securities, Gross Unrealized Loss1	(49)		(49)		(75)
Held-to-Maturity, Estimated Fair Value	50,315		50,315		32,544
Other Investments
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	378		378		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	379		379		$ 504
Other Investments
Available-for-Sale:
Available-for-Sale, Amortized Cost	3,795		3,795
Available-for-sale Securities, Gross Unrealized Gain1	0		0
Available-for-sale Securities, Gross Unrealized Loss1	0		0
Available-for-Sale, Estimated Fair Value	$ 3,795		$ 3,795