Securities - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017	Mar. 31, 2017
Amortized cost and fair value of securities available for sale
Amortized Cost	$ 1,106,028	$ 674,104	$ 2,946
Gross Unrealized Gains	4,546	7,872
Gross Unrealized Losses	(21,795)	(10,488)
Fair Value	1,088,779	671,488
Trust preferred securities
Amortized cost and fair value of securities available for sale
Amortized Cost	12,402	12,442
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2,001)	(3,054)
Fair Value	10,401	9,388
Other debt securities
Amortized cost and fair value of securities available for sale
Amortized Cost	23,555	17,106
Gross Unrealized Gains	94	260
Gross Unrealized Losses	(302)	(76)
Fair Value	23,347	17,290
Obligations of other U.S. Government agencies and corporations
Amortized cost and fair value of securities available for sale
Amortized Cost	3,545	3,554
Gross Unrealized Gains	19	40
Gross Unrealized Losses	(49)	(30)
Fair Value	3,515	3,564
Obligations of states and political subdivisions
Amortized cost and fair value of securities available for sale
Amortized Cost	217,847	228,589
Gross Unrealized Gains	3,787	6,161
Gross Unrealized Losses	(758)	(269)
Fair Value	220,876	234,481
Government agency mortgage backed securities \| Residential mortgage backed securities:
Amortized cost and fair value of securities available for sale
Amortized Cost	493,001	196,121
Gross Unrealized Gains	336	888
Gross Unrealized Losses	(10,212)	(3,059)
Fair Value	483,125	193,950
Government agency mortgage backed securities \| Commercial mortgage backed securities:
Amortized cost and fair value of securities available for sale
Amortized Cost	27,468	31,015
Gross Unrealized Gains	251	389
Gross Unrealized Losses	(530)	(234)
Fair Value	27,189	31,170
Government agency collateralized mortgage obligations \| Residential mortgage backed securities:
Amortized cost and fair value of securities available for sale
Amortized Cost	303,625	180,258
Gross Unrealized Gains	59	133
Gross Unrealized Losses	(7,679)	(3,752)
Fair Value	296,005	176,639
Government agency collateralized mortgage obligations \| Commercial mortgage backed securities:
Amortized cost and fair value of securities available for sale
Amortized Cost	24,585	5,019
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(264)	(14)
Fair Value	$ 24,321	$ 5,006

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.

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)
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Amount of unrealized gain before deducting unrealized loss on investments in debt securities classified as available-for-sale securities.

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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale debt securities.

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

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-Publisher FASB
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