Investment Securities Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Available-for-Sale:
Available-for-Sale, Amortized Cost	$ 1,450,396	$ 1,708,494
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	10,169	12,426
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	3,725	8,996
Available-for-Sale, Estimated Fair Value	1,456,840	1,711,924
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	600,665	575,186
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	12,854	11,566
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	(5,969)	(2,219)
Held-to-Maturity Estimated Fair Value	607,550	584,533
US Treasury Securities [Member]
Available-for-Sale:
Available-for-Sale, Amortized Cost	3,912
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	3
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	4
Available-for-Sale, Estimated Fair Value	3,911
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	521,079	725,408
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	712	895
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	1,610	5,370
Available-for-Sale, Estimated Fair Value	520,181	720,933
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	921,699	982,764
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	9,448	11,526
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	2,101	3,624
Available-for-Sale, Estimated Fair Value	929,046	990,666
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	376,480	353,176
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	7,686	5,563
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	(5,522)	(1,758)
Held-to-Maturity Estimated Fair Value	378,644	356,981
Other Investments
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	354	504
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	1	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held-to-Maturity Estimated Fair Value	355	504
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	173,785	188,941
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	5,103	5,949
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	(227)	(386)
Held-to-Maturity Estimated Fair Value	178,661	194,504
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	50,046	32,565
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	64	54
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	(220)	(75)
Held-to-Maturity Estimated Fair Value	49,890	32,544
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	156	322
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	1	5
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	1	2
Available-for-Sale, Estimated Fair Value	156	$ 325
Other Investments
Available-for-Sale:
Available-for-Sale, Amortized Cost	3,550
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	5
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	9
Available-for-Sale, Estimated Fair Value	$ 3,546