BASIS OF PRESENTATION (Narrative) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease assets	$ 24,664
Operating lease liabilities	26,333
Retained earnings	603,506		$ 579,252
Securities available for sale	2,400,460		2,450,709
Held to maturity securities	$ 88,089		$ 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)