INVESTMENT OPERATIONS - Summary of Major Categories of Net Investment Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									$ 2,506,125	$ 2,066,346	$ 1,959,064
Other investment expenses									167,223	143,290	135,601
Net investment income	$ 639,115	$ 631,955	$ 578,414	$ 489,418	$ 495,614	$ 477,011	$ 475,722	$ 474,709	2,338,902	1,923,056	1,823,463
Fixed maturities
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									2,043,183	1,610,768	1,547,346
Equity securities
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									35,282	40,506	38,838
Mortgage loans
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									322,206	298,387	270,749
Investment real estate
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									1,778	2,405	2,152
Short-term investments
Debt and Equity Securities, FV-NI [Line Items]
Investment income before other investment expenses									$ 103,676	$ 114,280	$ 99,979