CONSOLIDATED QUARTERLY RESULTS - UNAUDITED (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended								11 Months Ended	12 Months Ended
	Jan. 31, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Premiums and policy fees		$ 893,606	$ 855,088	$ 868,139	$ 860,586	$ 862,644	$ 834,544	$ 857,948	$ 852,795	$ 3,008,050	$ 3,477,419	$ 3,407,931
Reinsurance ceded		(376,641)	(325,120)	(342,898)	(316,076)	(345,555)	(322,229)	(336,605)	(310,327)	(1,154,978)	(1,360,735)	(1,314,716)
Net of reinsurance ceded		516,965	529,968	525,241	544,510	517,089	512,315	521,343	542,468	1,853,072	2,116,684	2,093,215
Net investment income		529,490	507,914	507,771	506,413	496,150	482,729	488,460	475,117	1,632,948	2,051,588	1,942,456
Realized investment gains (losses)		(18,701)	(64,191)	(54,471)	(47,037)	12,457	24,268	5,417	8,229	(163,882)	(196,142)	32,623
Net impairment losses recognized in earnings		(853)	(273)	(2,785)	(7,831)	(10,856)	(3,308)	(967)	(2,617)	(26,993)	(11,742)	(17,748)
Other income		115,139	110,970	111,311	109,242	102,147	107,642	102,148	103,716	388,531	446,662	415,653
Total revenues		1,142,040	1,084,388	1,087,067	1,105,297	1,116,987	1,123,646	1,116,401	1,126,913	3,710,669	4,418,792	4,483,947
Total benefits and expenses		1,055,950	973,538	961,299	992,948	996,572	990,567	947,740	955,071	3,310,827	3,983,735	3,889,950
Income before income tax	$ 1,182	86,090	110,850	125,768	112,349	120,415	133,079	168,661	171,842	399,842	435,057	593,997
Income tax expense (benefit)		(778,218)	28,308	41,500	36,935	48,148	39,785	56,541	56,494	131,543	(671,475)	200,968
Net income		$ 864,308	$ 82,542	$ 84,268	$ 75,414	$ 72,267	$ 93,294	$ 112,120	$ 115,348	$ 268,299	$ 1,106,532	$ 393,029