SUMMARY OF SIGINIFICANT 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	$ 1,639,441				$ 1,560,444				$ 1,639,441	$ 1,560,444
Cumulative effect adjustments													$ (93,916)
Statement of Financial Position [Abstract]
Deferred policy acquisition costs and value of business acquired	3,023,154				2,199,577				3,023,154	2,199,577	$ 2,019,829
Other liabilities	2,374,112				2,353,565				2,374,112	2,353,565
Income Statement [Abstract]
Other income	111,883	$ 113,530	$ 113,861	$ 114,411	$ 115,139	$ 110,970	$ 111,311	$ 109,242	453,685	446,662	415,653
Amortization of deferred policy acquisition costs and value of business acquired									225,808	78,221	149,064
Other operating expenses, net of reinsurance ceded									916,259	$ 948,244	$ 860,451
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustments													(83,364)
Retained Earnings | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustments													(93,900)
Retained Earnings | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustments													10,600
Previous Accounting Method | Accounting Standards Update 2014-09
Statement of Financial Position [Abstract]
Deferred policy acquisition costs and value of business acquired	2,881,600								2,881,600
Other liabilities	$ 2,110,500								2,110,500
Income Statement [Abstract]
Other income									454,400
Amortization of deferred policy acquisition costs and value of business acquired									177,000
Other operating expenses, net of reinsurance ceded									$ 968,100
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings													$ (93,900)
Scenario, Forecast | Accounting Standards Update 2016-02
Income Statement [Abstract]
Right-of-use asset												$ 21,500
Lease liability												21,500
Subsequent Event | Retained Earnings | Accounting Standards Update 2017-08
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustments												$ (51,200)