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BASIS OF PRESENTATION (Narrative) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease assets	$ 24,677
Operating lease liabilities	26,403
Retained earnings	670,816		$ 579,252
Securities available for sale	3,020,568		2,450,709
Held to maturity securities	$ (87,965)		$ (236,283)
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease assets		$ 25,900
Operating lease liabilities		27,600
Retained earnings		1,700
ASU 2017-12
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Securities available for sale		148,000
Held to maturity securities		$ 148,000