Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Available-for-Sale:
Available-for-Sale, Amortized Cost	$ 1,511,783	$ 1,450,396
Available-for-sale Securities, Gross Unrealized Gain1	21,980	10,169
Available-for-sale Securities, Gross Unrealized Loss1	(542)	(3,725)
Available-for-Sale, Estimated Fair Value	1,533,221	1,456,840
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	539,052	600,665
Held-to-maturity Securities, Gross Unrealized Gain1	21,621	12,854
Held-to-maturity Securities, Gross Unrealized Loss1	(6,625)	(5,969)
Held-to-Maturity, Estimated Fair Value	554,048	607,550
US Treasury Securities [Member]
Available-for-Sale:
Available-for-Sale, Amortized Cost	3,609	3,912
Available-for-sale Securities, Gross Unrealized Gain1	30	3
Available-for-sale Securities, Gross Unrealized Loss1	0	(4)
Available-for-Sale, Estimated Fair Value	3,639	3,911
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	439,488	521,079
Available-for-sale Securities, Gross Unrealized Gain1	1,819	712
Available-for-sale Securities, Gross Unrealized Loss1	(180)	(1,610)
Available-for-Sale, Estimated Fair Value	441,127	520,181
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	19,737	19,738
Held-to-maturity Securities, Gross Unrealized Gain1	315	0
Held-to-maturity Securities, Gross Unrealized Loss1	0	(102)
Held-to-Maturity, Estimated Fair Value	20,052	19,636
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,063,642	921,699
Available-for-sale Securities, Gross Unrealized Gain1	20,088	9,448
Available-for-sale Securities, Gross Unrealized Loss1	(361)	(2,101)
Available-for-Sale, Estimated Fair Value	1,083,369	929,046
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	300,018	356,742
Held-to-maturity Securities, Gross Unrealized Gain1	14,643	7,686
Held-to-maturity Securities, Gross Unrealized Loss1	(6,581)	(5,420)
Held-to-Maturity, Estimated Fair Value	308,080	359,008
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	124	156
Available-for-sale Securities, Gross Unrealized Gain1	1	1
Available-for-sale Securities, Gross Unrealized Loss1	(1)	(1)
Available-for-Sale, Estimated Fair Value	124	156
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	165,891	173,785
Held-to-maturity Securities, Gross Unrealized Gain1	6,048	5,103
Held-to-maturity Securities, Gross Unrealized Loss1	(30)	(227)
Held-to-Maturity, Estimated Fair Value	171,909	178,661
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	53,146	50,046
Held-to-maturity Securities, Gross Unrealized Gain1	614	64
Held-to-maturity Securities, Gross Unrealized Loss1	(14)	(220)
Held-to-Maturity, Estimated Fair Value	53,746	49,890
Other Investments
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	260	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	261	355
Other Investments
Available-for-Sale:
Available-for-Sale, Amortized Cost	4,920	3,550
Available-for-sale Securities, Gross Unrealized Gain1	42	5
Available-for-sale Securities, Gross Unrealized Loss1	0	(9)
Available-for-Sale, Estimated Fair Value	$ 4,962	$ 3,546

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