Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Available-for-Sale:
Available-for-Sale, Amortized Cost	$ 1,661,512	$ 1,624,976
Available-for-sale Securities, Gross Unrealized Gain1	8,728	6,782
Available-for-sale Securities, Gross Unrealized Loss1	(12,649)	(20,060)
Available-for-Sale, Estimated Fair Value	1,657,591	1,611,698
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	490,968	512,770
Held-to-maturity Securities, Gross Unrealized Gain1	12,414	10,667
Held-to-maturity Securities, Gross Unrealized Loss1	(9,457)	(10,164)
Held-to-Maturity, Estimated Fair Value	493,925	513,273
US Treasury Securities [Member]
Available-for-Sale:
Available-for-Sale, Amortized Cost	3,608	3,608
Available-for-sale Securities, Gross Unrealized Gain1	5	5
Available-for-sale Securities, Gross Unrealized Loss1	(1)	(1)
Available-for-Sale, Estimated Fair Value	3,612	3,612
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	411,096	397,411
Available-for-sale Securities, Gross Unrealized Gain1	449	343
Available-for-sale Securities, Gross Unrealized Loss1	(4,302)	(6,457)
Available-for-Sale, Estimated Fair Value	407,243	391,297
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	19,737	19,737
Held-to-maturity Securities, Gross Unrealized Gain1	0	0
Held-to-maturity Securities, Gross Unrealized Loss1	(82)	(162)
Held-to-Maturity, Estimated Fair Value	19,655	19,575
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,243,751	1,220,890
Available-for-sale Securities, Gross Unrealized Gain1	8,257	6,412
Available-for-sale Securities, Gross Unrealized Loss1	(8,345)	(13,601)
Available-for-Sale, Estimated Fair Value	1,243,663	1,213,701
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	264,427	279,578
Held-to-maturity Securities, Gross Unrealized Gain1	9,044	7,804
Held-to-maturity Securities, Gross Unrealized Loss1	(9,086)	(9,249)
Held-to-Maturity, Estimated Fair Value	264,385	278,133
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	106	116
Available-for-sale Securities, Gross Unrealized Gain1	1	1
Available-for-sale Securities, Gross Unrealized Loss1	(1)	(1)
Available-for-Sale, Estimated Fair Value	106	116
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	153,679	160,192
Held-to-maturity Securities, Gross Unrealized Gain1	3,121	2,723
Held-to-maturity Securities, Gross Unrealized Loss1	(222)	(542)
Held-to-Maturity, Estimated Fair Value	156,578	162,373
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	52,918	53,032
Held-to-maturity Securities, Gross Unrealized Gain1	248	139
Held-to-maturity Securities, Gross Unrealized Loss1	(67)	(211)
Held-to-Maturity, Estimated Fair Value	53,099	52,960
Other Investments
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	207	231
Held-to-maturity Securities, Gross Unrealized Gain1	1	1
Held-to-maturity Securities, Gross Unrealized Loss1	0	0
Held-to-Maturity, Estimated Fair Value	208	232
Other Investments
Available-for-Sale:
Available-for-Sale, Amortized Cost	2,951	2,951
Available-for-sale Securities, Gross Unrealized Gain1	16	21
Available-for-sale Securities, Gross Unrealized Loss1	0	0
Available-for-Sale, Estimated Fair Value	$ 2,967	$ 2,972

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