Investment Securities - Amortized cost and fair value for available for sale securities - (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Investment securities available for sale
Amortized cost	$ 1,598,898	$ 1,655,107	$ 1,315,802
Gross Unrealized Gains	1,229	5,903	9,505
Gross Unrealized Losses	(48,846)	(12,817)	(5,853)
Fair Value	1,551,281	1,648,193	1,319,454
Government-sponsored entities debt
Investment securities available for sale
Amortized cost	54,397	86,535	86,521
Gross Unrealized Gains		51	72
Gross Unrealized Losses	(1,852)	(1,077)	(642)
Fair Value	52,545	85,509	85,951
State and municipal obligations
Investment securities available for sale
Amortized cost	207,723	216,812	199,898
Gross Unrealized Gains	1,114	3,749	4,584
Gross Unrealized Losses	(2,613)	(124)	(188)
Fair Value	206,224	220,437	204,294
Mortgage-backed securities
Investment securities available for sale
Amortized cost	1,336,778	1,350,200	1,027,827
Gross Unrealized Gains	115	2,103	4,673
Gross Unrealized Losses	(44,381)	(11,616)	(5,023)
Fair Value	1,292,512	1,340,687	1,027,477
Common Stock
Investment securities available for sale
Amortized cost		1,560	1,556
Gross Unrealized Gains			176
Fair Value		1,560	1,732
Mortgage-backed securities issued by private label holdings
Investment securities available for sale
Fair Value	$ 0	$ 0	$ 0

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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 investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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