Investment Securities (Summary of Amortized Cost and Fair Value of Available-for-Sale Investment Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment Securities Available-for-Sale
Amortized Cost	$ 1,976,646	$ 1,256,721
Unrealized Gains	4,040	1,566
Unrealized Losses	17,140	23,323
Available-for-sale, at fair value	1,963,546	1,234,964
U.S. Treasury securities
Investment Securities Available-for-Sale
Amortized Cost		5,788
Unrealized Gains		5
Unrealized Losses		0
Available-for-sale, at fair value		5,793
Government sponsored agencies
Investment Securities Available-for-Sale
Amortized Cost	203,099	216,890
Unrealized Gains	765	189
Unrealized Losses	948	2,068
Available-for-sale, at fair value	202,916	215,011
State and political subdivisions
Investment Securities Available-for-Sale
Amortized Cost	350,088	311,704
Unrealized Gains	310	163
Unrealized Losses	4,428	11,779
Available-for-sale, at fair value	345,970	300,088
Residential mortgage-backed securities
Investment Securities Available-for-Sale
Amortized Cost	151,752	276,162
Unrealized Gains	5	112
Unrealized Losses	1,626	3,992
Available-for-sale, at fair value	150,131	272,282
Collateralized mortgage obligations
Investment Securities Available-for-Sale
Amortized Cost	1,042,240	323,965
Unrealized Gains	89	63
Unrealized Losses	8,484	4,003
Available-for-sale, at fair value	1,033,845	320,025
Corporate bonds
Investment Securities Available-for-Sale
Amortized Cost	193,230	90,859
Unrealized Gains	1,156	16
Unrealized Losses	1,592	1,401
Available-for-sale, at fair value	192,794	89,474
Preferred stock and trust preferred securities
Investment Securities Available-for-Sale
Amortized Cost	36,237	31,353
Unrealized Gains	1,715	1,018
Unrealized Losses	62	80
Available-for-sale, at fair value	$ 37,890	$ 32,291

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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

+ 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 (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount of investment in debt and equity 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=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: 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=82896770&loc=d3e26610-111562

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+ References

Reference 1: 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

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