Investment Securities - Amortized cost and fair value for available for sale securities - (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Investment securities available for sale
Amortized cost	$ 1,669,833	$ 1,655,107	$ 1,377,433
Gross Unrealized Gains	2,480	5,903	9,927
Gross Unrealized Losses	(31,476)	(12,817)	(7,572)
Fair Value	1,640,837	1,648,193	1,379,788
Government-sponsored entities debt
Investment securities available for sale
Amortized cost	91,483	86,535	93,995
Gross Unrealized Gains		51	25
Gross Unrealized Losses	(1,737)	(1,077)	(721)
Fair Value	89,746	85,509	93,299
State and municipal obligations
Investment securities available for sale
Amortized cost	224,994	216,812	196,241
Gross Unrealized Gains	2,232	3,749	4,183
Gross Unrealized Losses	(1,540)	(124)	(140)
Fair Value	225,686	220,437	200,284
Mortgage-backed securities. issued by Government sponsored entities
Investment securities available for sale
Amortized cost	1,353,356	1,350,200	1,084,764
Gross Unrealized Gains	248	2,103	5,095
Gross Unrealized Losses	(28,199)	(11,616)	(6,420)
Fair Value	1,325,405	1,340,687	1,083,439
Corporate securities
Investment securities available for sale
Amortized cost		1,560	2,433
Gross Unrealized Gains			624
Gross Unrealized Losses			(291)
Fair Value		1,560	2,766
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 investments in debt securities classified as available-for-sale.

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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

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- 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=82897076&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

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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=75030427&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=82896770&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=82897076&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=75019621&loc=d3e62557-112803

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