SEGMENT INFORMATION - Gross premiums written (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 6,910,065	$ 5,556,273	$ 4,970,208
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	606,452	529,425	465,980
Ireland
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	1,805,882	1,569,956	1,650,229
U.S.
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	2,811,537	2,814,933	2,562,789
Lloyd's of London
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 1,686,194	$ 641,959	$ 291,210