Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Schedule of Available for Sale and Held-to-Maturity Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains	$ 57	$ 6
Available-for-sale Securities, Gross Realized Losses	78	$ 0
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,548,217		$ 1,450,396
Available-for-sale Securities, Gross Unrealized Gain1	16,321		10,169
Available-for-sale Securities, Gross Unrealized Loss1	(1,188)		(3,725)
Available-for-Sale, Estimated Fair Value	1,563,350		1,456,840
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	581,390		600,665
Held-to-maturity Securities, Gross Unrealized Gain1	17,734		12,854
Held-to-maturity Securities, Gross Unrealized Loss1	(5,220)		(5,969)
Held-to-Maturity, Estimated Fair Value	593,904		607,550
US Treasury Securities [Member]
Available-for-Sale:
Available-for-Sale, Amortized Cost	3,911
Available-for-sale Securities, Gross Unrealized Gain1	24
Available-for-sale Securities, Gross Unrealized Loss1	0
Available-for-Sale, Estimated Fair Value	3,935
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	530,037		521,079
Available-for-sale Securities, Gross Unrealized Gain1	1,719		712
Available-for-sale Securities, Gross Unrealized Loss1	(324)		(1,610)
Available-for-Sale, Estimated Fair Value	531,432		520,181
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	19,737		19,738
Held-to-maturity Securities, Gross Unrealized Gain1	248		0
Held-to-maturity Securities, Gross Unrealized Loss1	0		(102)
Held-to-Maturity, Estimated Fair Value	19,985		19,636
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,010,675		921,699
Available-for-sale Securities, Gross Unrealized Gain1	14,550		9,448
Available-for-sale Securities, Gross Unrealized Loss1	(862)		(2,101)
Available-for-Sale, Estimated Fair Value	1,024,363		929,046
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	341,434		356,742
Held-to-maturity Securities, Gross Unrealized Gain1	11,640		7,686
Held-to-maturity Securities, Gross Unrealized Loss1	(5,145)		(5,420)
Held-to-Maturity, Estimated Fair Value	347,929		359,008
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	144		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	143		156
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	167,975		173,785
Held-to-maturity Securities, Gross Unrealized Gain1	5,553		5,103
Held-to-maturity Securities, Gross Unrealized Loss1	(14)		(227)
Held-to-Maturity, Estimated Fair Value	173,514		178,661
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	51,918		50,046
Held-to-maturity Securities, Gross Unrealized Gain1	292		64
Held-to-maturity Securities, Gross Unrealized Loss1	(61)		(220)
Held-to-Maturity, Estimated Fair Value	52,149		49,890
Other Investments
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	326		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	327		355
Other Investments
Available-for-Sale:
Available-for-Sale, Amortized Cost	3,450		3,550
Available-for-sale Securities, Gross Unrealized Gain1	27		5
Available-for-sale Securities, Gross Unrealized Loss1	0		(9)
Available-for-Sale, Estimated Fair Value	$ 3,477		$ 3,546

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