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	$ 971,629	$ 878,182	$ 941,868	$ 889,166	$ 893,606	$ 855,088	$ 868,139	$ 860,586	$ 3,680,845	$ 3,477,419	$ 3,407,931
Reinsurance ceded	(379,242)	(269,335)	(390,941)	(345,423)	(376,641)	(325,120)	(342,898)	(316,076)	(1,384,941)	(1,360,735)	(1,314,716)
Net of reinsurance ceded	592,387	608,847	550,927	543,743	516,965	529,968	525,241	544,510	2,295,904	2,116,684	2,093,215
Net investment income	674,286	672,139	616,462	520,863	529,490	507,914	507,771	506,413	2,483,750	2,051,588	1,942,456
Realized investment gains (losses)	(68,450)	(47,334)	(37,337)	(9,540)	(18,701)	(64,191)	(54,471)	(47,037)	(192,385)	(196,142)	32,623
Net impairment losses recognized in earnings	26,060	14	5	3,645	853	273	2,785	7,831	29,724	11,742	17,748
Other income	111,883	113,530	113,861	114,411	115,139	110,970	111,311	109,242	453,685	446,662	415,653
Total revenues	1,284,046	1,347,168	1,243,908	1,165,832	1,142,040	1,084,388	1,087,067	1,105,297	5,040,954	4,418,792	4,483,947
Total benefits and expenses	1,228,317	1,210,449	1,145,136	1,074,034	1,055,950	973,538	961,299	992,948	4,657,936	3,983,735	3,889,950
Income before income tax	55,729	136,719	98,772	91,798	86,090	110,850	125,768	112,349	383,018	435,057	593,997
Income tax expense (benefit)	19,075	26,619	17,277	17,686	(778,218)	28,308	41,500	36,935	80,657	(671,475)	200,968
Net income	$ 36,654	$ 110,100	$ 81,495	$ 74,112	$ 864,308	$ 82,542	$ 84,268	$ 75,414	$ 302,361	$ 1,106,532	$ 393,029