Statutory Information and Restrictions - Reconciliations of Statutory Capital, Surplus and Net Income (Details) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	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, 2019	Dec. 31, 2018	Dec. 31, 2017
Insurance [Abstract]
Statutory capital and surplus of insurance subsidiaries	$ 868,839				$ 903,564								$ 868,839	$ 903,564
Increase (decrease) due to:
Deferred policy acquisition costs	276,668				298,742								276,668	298,742
Deposit asset on reinsurance	2,346,166				0								2,346,166	0
Annuity reserves ceded	(2,239,717)				0								(2,239,717)	0
Difference in policyholder reserves	209,127				142,601								209,127	142,601
Goodwill	49,079				47,396								49,079	47,396
Intangible assets, net	177,217				0								177,217	0
Investment fair value adjustments on fixed maturity securities	397,762				142,512								397,762	142,512
Difference in investment reserves	102,380				105,430								102,380	105,430
Federal income tax liability	(178,026)				(115,667)								(178,026)	(115,667)
Net funded status of benefit plans	(13,690)				(15,495)								(13,690)	(15,495)
Non-admitted assets and other, net	(53,801)				20,412								(53,801)	20,412
Shareholders' equity of parent company and non-insurance subsidiaries	8,306				8,795								8,306	8,795
Parent company short-term and long-term debt	(383,025)				(247,740)								(383,025)	(247,740)
Total shareholders' equity	1,567,285				1,290,550				$ 1,501,573				1,567,285	1,290,550	$ 1,501,573
Statutory net income of insurance subsidiaries													62,316	45,977	82,587
Net loss of non-insurance companies													(9,537)	(9,755)	(4,496)
Interest expense													(14,272)	(11,892)	(11,836)
Tax benefit of interest expense and other parent company current tax adjustments													8,993	121	5,654
Combined net income													47,500	24,451	71,909
Increase (decrease) due to:
Deferred policy acquisition costs													2,101	1,015	9,385
Intangible asset amortization expense													(8,790)	0	0
Policyholder benefits													117,369	26,318	30,609
Federal income tax (expense) benefit													(23,492)	3,020	84,198
Investment reserves													88,627	(31,529)	(20,966)
Other adjustments, net													(38,872)	(4,932)	(5,676)
Net income	$ 33,001	$ 25,454	$ 93,822	$ 32,166	$ (20,257)	$ 12,528	$ 5,917	$ 20,155	$ 125,329	$ 26,551	$ 2,261	$ 15,318	$ 184,443	$ 18,343	$ 169,459