INVESTMENT OPERATIONS - Summary of Major Categories of Net Investment Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									$ 3,081,471	$ 3,004,422	$ 2,506,125
Investment expenses									198,508	185,592	167,223
Net investment income	$ 651,377	$ 738,171	$ 740,435	$ 752,980	$ 752,503	$ 739,711	$ 685,194	$ 641,422	2,882,963	2,818,830	2,338,902
Fixed maturities
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									2,475,295	2,465,902	2,043,183
Equity securities
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									24,492	30,647	35,282
Commercial mortgage loans
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									442,752	388,656	322,206
Investment real estate
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									989	1,045	1,778
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									$ 137,943	$ 118,172	$ 103,676