FAIR VALUE MEASUREMENTS - Carrying Value and Fair Value of Debt (Details) - Level 2 - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Carrying Amount
Fair Value And Carrying Value By Balance Sheet Grouping
Total debt	$ 3,839,981	$ 4,714,442
Carrying Amount | Fixed-rate debt
Fair Value And Carrying Value By Balance Sheet Grouping
Total debt	3,273,334	3,546,396
Carrying Amount | Variable-rate debt
Fair Value And Carrying Value By Balance Sheet Grouping
Total debt	566,647	1,168,046
Fair Value
Fair Value And Carrying Value By Balance Sheet Grouping
Total debt	3,616,808	4,536,877
Fair Value | Fixed-rate debt
Fair Value And Carrying Value By Balance Sheet Grouping
Total debt	3,046,533	3,364,549
Fair Value | Variable-rate debt
Fair Value And Carrying Value By Balance Sheet Grouping
Total debt	$ 570,275	$ 1,172,328