CONSOLIDATED QUARTERLY RESULTS - UNAUDITED (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
Quarterly Financial Information Disclosure [Abstract]
Premiums and policy fees	$ 965,275	$ 871,892	$ 935,928	$ 883,413	$ 888,134	$ 849,743	$ 862,906	$ 855,579	$ 3,656,508	$ 3,456,362	$ 3,389,419
Reinsurance ceded	(378,112)	(267,910)	(391,864)	(345,624)	(377,261)	(326,572)	(344,208)	(319,055)	(1,383,510)	(1,367,096)	(1,330,723)
Net of reinsurance ceded	587,163	603,982	544,064	537,789	510,873	523,171	518,698	536,524	2,272,998	2,089,266	2,058,696
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
Realized investment gains (losses)	(24,005)	(46,866)	(32,583)	(40,725)	8,912	23,452	(10,259)	(38,059)	(173,903)	(25,066)	122,672
Net impairment losses recognized in earnings	(26,060)	(14)	(5)	(3,645)	(853)	(273)	(2,785)	(5,201)	(29,724)	(9,112)	(17,748)
Other income	79,685	80,906	79,754	80,674	81,910	82,031	82,087	79,383	321,019	325,411	288,878
Revenues	1,255,898	1,269,963	1,169,644	1,063,511	1,096,456	1,105,392	1,063,463	1,047,356	4,759,016	4,312,667	4,293,709
Total benefits and expenses	1,186,310	1,174,793	1,108,750	1,041,575	1,018,360	939,410	930,177	960,712	4,511,428	3,848,659	3,771,028
Income before income tax	69,588	95,170	60,894	21,936	78,096	165,982	133,286	86,644	247,588	464,008	522,681
Income tax expense	24,728	16,646	8,626	3,661	(839,384)	49,016	43,654	28,305	53,661	(718,409)	170,073
Net income	$ 44,860	$ 78,524	$ 52,268	$ 18,275	$ 917,480	$ 116,966	$ 89,632	$ 58,339	$ 193,927	$ 1,182,417	$ 352,608