ACCOUNTING POLICIES - Recently Issued or Adopted Accounting Standards (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle
Operating lease assets	$ 359,025		$ 0
Operating lease liabilities	$ 214,046		0
Cumulative effect adjustment to retained earnings			637	$ 30,643
Minimum
New Accounting Pronouncements or Change in Accounting Principle
Operating lease, discount rate		6.15%
Maximum
New Accounting Pronouncements or Change in Accounting Principle
Operating lease, discount rate		7.60%
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle
Operating lease assets		$ 361,700
Operating lease liabilities		$ 216,900
Cumulative effect adjustment to retained earnings			$ 600