Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Available-for-Sale:
Available-for-Sale, Amortized Cost	$ 2,076,114	$ 1,624,976
Available-for-sale Securities, Gross Unrealized Gain1	11,971	6,782
Available-for-sale Securities, Gross Unrealized Loss1	(7,891)	(20,060)
Available-for-Sale, Estimated Fair Value	2,080,194	1,611,698
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	529,442	512,770
Held-to-maturity Securities, Gross Unrealized Gain1	15,248	10,667
Held-to-maturity Securities, Gross Unrealized Loss1	(9,545)	(10,164)
Held-to-Maturity, Estimated Fair Value	535,145	513,273
US Treasury Securities [Member]
Available-for-Sale:
Available-for-Sale, Amortized Cost	3,607	3,608
Available-for-sale Securities, Gross Unrealized Gain1	2	5
Available-for-sale Securities, Gross Unrealized Loss1	(2)	(1)
Available-for-Sale, Estimated Fair Value	3,607	3,612
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	676,485	397,411
Available-for-sale Securities, Gross Unrealized Gain1	1,647	343
Available-for-sale Securities, Gross Unrealized Loss1	(2,814)	(6,457)
Available-for-Sale, Estimated Fair Value	675,318	391,297
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	19,737	19,737
Held-to-maturity Securities, Gross Unrealized Gain1	28	0
Held-to-maturity Securities, Gross Unrealized Loss1	(5)	(162)
Held-to-Maturity, Estimated Fair Value	19,760	19,575
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,217,701	1,220,890
Available-for-sale Securities, Gross Unrealized Gain1	10,001	6,412
Available-for-sale Securities, Gross Unrealized Loss1	(4,765)	(13,601)
Available-for-Sale, Estimated Fair Value	1,222,937	1,213,701
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	268,218	279,578
Held-to-maturity Securities, Gross Unrealized Gain1	10,714	7,804
Held-to-maturity Securities, Gross Unrealized Loss1	(9,360)	(9,249)
Held-to-Maturity, Estimated Fair Value	269,572	278,133
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	108,577	116
Available-for-sale Securities, Gross Unrealized Gain1	212	1
Available-for-sale Securities, Gross Unrealized Loss1	(304)	(1)
Available-for-Sale, Estimated Fair Value	108,485	116
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	188,486	160,192
Held-to-maturity Securities, Gross Unrealized Gain1	4,172	2,723
Held-to-maturity Securities, Gross Unrealized Loss1	(141)	(542)
Held-to-Maturity, Estimated Fair Value	192,517	162,373
Corporate securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	66,793
Available-for-sale Securities, Gross Unrealized Gain1	97
Available-for-sale Securities, Gross Unrealized Loss1	(6)
Available-for-Sale, Estimated Fair Value	66,884
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	52,804	53,032
Held-to-maturity Securities, Gross Unrealized Gain1	333	139
Held-to-maturity Securities, Gross Unrealized Loss1	(39)	(211)
Held-to-Maturity, Estimated Fair Value	53,098	52,960
Other Investments
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	197	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	198	232
Other Investments
Available-for-Sale:
Available-for-Sale, Amortized Cost	2,951	2,951
Available-for-sale Securities, Gross Unrealized Gain1	12	21
Available-for-sale Securities, Gross Unrealized Loss1	0	0
Available-for-Sale, Estimated Fair Value	$ 2,963	$ 2,972

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of debt securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 2
-Subparagraph (b)-(g)
-URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27232-111563

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.6)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_HeldToMaturitySecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of investments in debt securities classified as held-to-maturity.

+ References

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesUnclassifiedAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_OtherInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtSecurityAxis=us-gaap_OtherDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: