Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Available-for-Sale:
Available-for-Sale, Amortized Cost	$ 1,480,501	$ 1,450,396
Available-for-sale Securities, Gross Unrealized Gain1	25,220	10,169
Available-for-sale Securities, Gross Unrealized Loss1	(140)	(3,725)
Available-for-Sale, Estimated Fair Value	1,505,581	1,456,840
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	556,247	600,665
Held-to-maturity Securities, Gross Unrealized Gain1	24,013	12,854
Held-to-maturity Securities, Gross Unrealized Loss1	(5,778)	(5,969)
Held-to-Maturity, Estimated Fair Value	574,482	607,550
US Treasury Securities [Member]
Available-for-Sale:
Available-for-Sale, Amortized Cost	3,909	3,912
Available-for-sale Securities, Gross Unrealized Gain1	44	3
Available-for-sale Securities, Gross Unrealized Loss1	0	(4)
Available-for-Sale, Estimated Fair Value	3,953	3,911
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	450,544	521,079
Available-for-sale Securities, Gross Unrealized Gain1	2,882	712
Available-for-sale Securities, Gross Unrealized Loss1	(21)	(1,610)
Available-for-Sale, Estimated Fair Value	453,405	520,181
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	19,737	19,738
Held-to-maturity Securities, Gross Unrealized Gain1	483	0
Held-to-maturity Securities, Gross Unrealized Loss1	0	(102)
Held-to-Maturity, Estimated Fair Value	20,220	19,636
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,022,462	921,699
Available-for-sale Securities, Gross Unrealized Gain1	22,252	9,448
Available-for-sale Securities, Gross Unrealized Loss1	(117)	(2,101)
Available-for-Sale, Estimated Fair Value	1,044,597	929,046
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	320,620	356,742
Held-to-maturity Securities, Gross Unrealized Gain1	15,925	7,686
Held-to-maturity Securities, Gross Unrealized Loss1	(5,777)	(5,420)
Held-to-Maturity, Estimated Fair Value	330,768	359,008
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	136	156
Available-for-sale Securities, Gross Unrealized Gain1	1	1
Available-for-sale Securities, Gross Unrealized Loss1	(2)	(1)
Available-for-Sale, Estimated Fair Value	135	156
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	164,334	173,785
Held-to-maturity Securities, Gross Unrealized Gain1	7,024	5,103
Held-to-maturity Securities, Gross Unrealized Loss1	(1)	(227)
Held-to-Maturity, Estimated Fair Value	171,357	178,661
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	51,262	50,046
Held-to-maturity Securities, Gross Unrealized Gain1	580	64
Held-to-maturity Securities, Gross Unrealized Loss1	0	(220)
Held-to-Maturity, Estimated Fair Value	51,842	49,890
Other Investments
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	294	354
Held-to-maturity Securities, Gross Unrealized Gain1	1	1
Held-to-maturity Securities, Gross Unrealized Loss1	0	0
Held-to-Maturity, Estimated Fair Value	295	355
Other Investments
Available-for-Sale:
Available-for-Sale, Amortized Cost	3,450	3,550
Available-for-sale Securities, Gross Unrealized Gain1	41	5
Available-for-sale Securities, Gross Unrealized Loss1	0	(9)
Available-for-Sale, Estimated Fair Value	$ 3,491	$ 3,546

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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