CONSOLIDATED STATEMENTS OF INCOME - 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
Revenues
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):
Derivative financial instruments									60,988	(305,828)	(40,288)
All other investments									(223,649)	121,428	90,659
Other-than-temporary impairment losses									(56,578)	(3,962)	(32,075)
Portion recognized in other comprehensive income (before taxes)									26,854	(7,780)	14,327
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
Benefits and expenses
Benefits and settlement expenses, net of reinsurance ceded: (2018 - $1,191,978; 2017 - $1,242,797; 2016 - $1,181,960)									3,515,869	2,957,270	2,880,435
Amortization of deferred policy acquisition costs and value of business acquired									225,808	78,221	149,064
Other operating expenses, net of reinsurance ceded: (2018 - $205,352; 2017 - $222,963; 2016 - $207,197)									916,259	948,244	860,451
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 (benefit) expense
Current									95,561	26,252	(46,719)
Deferred									(14,904)	(697,727)	247,687
Total 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