Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Available-for-Sale:
Amortized Cost	$ 2,110,200	$ 1,624,976
Gross Unrealized Gains	9,616	6,782
Gross Unrealized Losses	(8,119)	(20,060)
Estimated Fair Value	2,111,697	1,611,698
Held-to-Maturity:
Amortized Cost	505,985	512,770
Gross Unrealized Gains	14,767	10,667
Gross Unrealized Losses	(10,347)	(10,164)
Estimated Fair Value	510,405	513,273
U.S. Treasury notes
Available-for-Sale:
Amortized Cost	3,206	3,608
Gross Unrealized Gains	1	5
Gross Unrealized Losses	(2)	(1)
Estimated Fair Value	3,205	3,612
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost	508,449	397,411
Gross Unrealized Gains	338	343
Gross Unrealized Losses	(3,209)	(6,457)
Estimated Fair Value	505,578	391,297
Held-to-Maturity:
Amortized Cost	19,737	19,737
Gross Unrealized Gains	43	0
Gross Unrealized Losses	(19)	(162)
Estimated Fair Value	19,761	19,575
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Amortized Cost	1,431,125	1,220,890
Gross Unrealized Gains	8,829	6,412
Gross Unrealized Losses	(4,716)	(13,601)
Estimated Fair Value	1,435,238	1,213,701
Held-to-Maturity:
Amortized Cost	243,380	279,578
Gross Unrealized Gains	10,725	7,804
Gross Unrealized Losses	(10,172)	(9,249)
Estimated Fair Value	243,933	278,133
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost	97,715	116
Gross Unrealized Gains	313	1
Gross Unrealized Losses	(190)	(1)
Estimated Fair Value	97,838	116
State, county and municipal securities
Held-to-Maturity:
Amortized Cost	180,981	160,192
Gross Unrealized Gains	3,726	2,723
Gross Unrealized Losses	(123)	(542)
Estimated Fair Value	184,584	162,373
Corporate securities
Available-for-Sale:
Amortized Cost	66,754
Gross Unrealized Gains	126
Gross Unrealized Losses	(2)
Estimated Fair Value	66,878
Held-to-Maturity:
Amortized Cost	61,704	53,032
Gross Unrealized Gains	272	139
Gross Unrealized Losses	(33)	(211)
Estimated Fair Value	61,943	52,960
Other investments
Available-for-Sale:
Amortized Cost	2,951	2,951
Gross Unrealized Gains	9	21
Gross Unrealized Losses	0	0
Estimated Fair Value	2,960	2,972
Other Investments
Held-to-Maturity:
Amortized Cost	183	231
Gross Unrealized Gains	1	1
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 184	$ 232

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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