INVESTMENT OPERATIONS - Summary of Major Categories of Net Investment Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									$ 3,004,422	$ 2,506,125	$ 2,066,346
Investment expenses									185,592	167,223	143,290
Net investment income	$ 752,503	$ 739,711	$ 685,194	$ 641,422	$ 639,115	$ 631,955	$ 578,414	$ 489,418	2,818,830	2,338,902	1,923,056
Fixed maturities
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									2,465,902	2,043,183	1,610,768
Equity securities
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									30,647	35,282	40,506
Mortgage loans
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									388,656	322,206	298,387
Investment real estate
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									1,045	1,778	2,405
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									$ 118,172	$ 103,676	$ 114,280