SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounting Pronouncements (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	Jan. 01, 2019	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 1,031,465				$ 916,971				$ 1,031,465	$ 916,971
Cumulative effect adjustments													$ (89,985)
Statement of Financial Position [Abstract]
Deferred policy acquisition costs and value of business acquired	3,026,330				2,205,401				3,026,330	2,205,401	$ 2,024,524
Other liabilities	1,939,718				1,859,254				1,939,718	1,859,254
Income Statement [Abstract]
Other income	79,685	$ 80,906	$ 79,754	$ 80,674	$ 81,910	$ 82,031	$ 82,087	$ 79,383	321,019	325,411	288,878
Amortization of deferred policy acquisition costs and value of business acquired									226,066	79,443	150,298
Other operating expenses, net of reinsurance ceded									774,110	$ 814,211	$ 744,004
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustments													(79,433)
Accounting Standards Update 2014-09 | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustments													(90,000)
Accounting Standards Update 2016-01 | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustments													10,600
Accounting Standards Update 2016-02 | Scenario, Forecast
Income Statement [Abstract]
Right-of-use asset												$ 21,500
Lease liability												21,500
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings													$ (90,000)
Calculated under Revenue Guidance in Effect before Topic 606 | Accounting Standards Update 2014-09
Statement of Financial Position [Abstract]
Deferred policy acquisition costs and value of business acquired	2,887,300								2,887,300
Other liabilities	$ 1,683,300								1,683,300
Income Statement [Abstract]
Other income									322,100
Amortization of deferred policy acquisition costs and value of business acquired									178,100
Other operating expenses, net of reinsurance ceded									$ 825,100
Subsequent Event | Accounting Standards Update 2017-08 | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustments												$ (50,800)