Investment Securities Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Available-for-Sale:
Available-for-Sale, Amortized Cost	$ 1,624,976	$ 1,450,396
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	6,782	10,169
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(20,060)	(3,725)
Available-for-Sale, Estimated Fair Value	1,611,698	1,456,840
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	512,770	600,665
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	10,667	12,854
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	(10,164)	(5,969)
Held-to-Maturity Estimated Fair Value	513,273	607,550
US Treasury Securities [Member]
Available-for-Sale:
Available-for-Sale, Amortized Cost	3,608	3,912
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	5	3
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(1)	(4)
Available-for-Sale, Estimated Fair Value	3,612	3,911
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	397,411	521,079
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	343	712
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(6,457)	(1,610)
Available-for-Sale, Estimated Fair Value	391,297	520,181
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	19,737
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	(162)
Held-to-Maturity Estimated Fair Value	19,575
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,220,890	921,699
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	6,412	9,448
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(13,601)	(2,101)
Available-for-Sale, Estimated Fair Value	1,213,701	929,046
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	279,578	376,480
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	7,804	7,686
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	(9,249)	(5,522)
Held-to-Maturity Estimated Fair Value	278,133	378,644
Other Investments
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	231	354
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	1	1
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0	0
Held-to-Maturity Estimated Fair Value	232	355
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	160,192	173,785
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	2,723	5,103
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	(542)	(227)
Held-to-Maturity Estimated Fair Value	162,373	178,661
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	53,032	50,046
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	139	64
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	(211)	(220)
Held-to-Maturity Estimated Fair Value	52,960	49,890
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	116	156
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	1	1
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(1)	(1)
Available-for-Sale, Estimated Fair Value	116	156
Other Investments
Available-for-Sale:
Available-for-Sale, Amortized Cost	2,951	3,550
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	21	5
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	0	(9)
Available-for-Sale, Estimated Fair Value	$ 2,972	$ 3,546