Business Segments - Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue:
Premium	$ 860,750	$ 819,217	$ 2,564,835	$ 2,456,462
Net investment income	221,627	213,872	658,279	634,930
Other income	393	331	1,104	1,140
Total revenue	1,082,770	1,033,420	3,224,218	3,092,532
Expenses:
Policy benefits	567,856	551,219	1,706,122	1,665,410
Required interest on reserves	0	0	0	0
Required interest on DAC	0	0	0	0
Amortization of acquisition costs	129,492	122,334	388,189	370,363
Commissions, premium taxes, and non-deferred acquisition costs	69,632	67,863	208,698	198,011
Insurance administrative expense	59,402	52,426	170,150	155,751
Parent expense	4,245	2,330	9,384	7,228
Stock-based compensation expense	10,412	8,263	29,969	24,809
Interest expense	22,433	20,970	66,466	62,825
Total benefits and expenses	863,472	825,405	2,578,978	2,484,397
Subtotal	219,298	208,015	645,240	608,135
Non-operating items	5,168	1,392	5,168	3,486
Measure of segment profitability (pretax)	224,466	209,407	650,408	611,621
Deduct applicable income taxes	(41,744)	(63,342)	(120,456)	(184,703)
Net operating income	182,722	146,065	529,952	426,918
Add back income taxes applicable to segment profitability	41,744	63,342	120,456	184,703
Add (deduct) realized investment gains (losses)	1,032	12,595	14,796	6,142
Add (deduct) guaranty fund assessments		(1,392)		(1,392)
Add (deduct) administrative settlements	(3,590)		(3,590)	(2,094)
Add (deduct) non-operating costs	(1,578)		(1,578)
Income before income taxes	220,330	220,610	660,036	614,277
Operating Segments [Member] | Life [Member]
Revenue:
Premium	605,547	576,223	1,806,384	1,725,896
Total revenue	605,547	576,223	1,806,384	1,725,896
Expenses:
Policy benefits	396,701	386,445	1,196,616	1,166,289
Required interest on reserves	(160,058)	(152,862)	(474,373)	(452,339)
Required interest on DAC	48,907	46,893	145,126	139,042
Amortization of acquisition costs	103,792	98,268	312,168	296,646
Commissions, premium taxes, and non-deferred acquisition costs	47,609	44,748	142,253	132,094
Total benefits and expenses	436,951	423,492	1,321,790	1,281,732
Subtotal	168,596	152,731	484,594	444,164
Measure of segment profitability (pretax)	168,596	152,731	484,594	444,164
Operating Segments [Member] | Health [Member]
Revenue:
Premium	255,201	242,991	758,439	730,557
Total revenue	255,201	242,991	758,439	730,557
Expenses:
Policy benefits	162,574	155,774	483,654	470,104
Required interest on reserves	(20,965)	(19,617)	(62,095)	(57,859)
Required interest on DAC	6,136	5,881	18,228	17,530
Amortization of acquisition costs	25,162	23,458	74,311	71,801
Commissions, premium taxes, and non-deferred acquisition costs	22,041	21,746	66,502	64,599
Total benefits and expenses	194,948	187,242	580,600	566,175
Subtotal	60,253	55,749	177,839	164,382
Measure of segment profitability (pretax)	60,253	55,749	177,839	164,382
Operating Segments [Member] | Annuity [Member]
Revenue:
Premium	2	3	12	9
Total revenue	2	3	12	9
Expenses:
Policy benefits	8,581	9,000	25,852	26,923
Required interest on reserves	(11,831)	(12,433)	(35,733)	(37,245)
Required interest on DAC	145	172	448	524
Amortization of acquisition costs	538	608	1,710	1,916
Commissions, premium taxes, and non-deferred acquisition costs	5	7	19	25
Total benefits and expenses	(2,562)	(2,646)	(7,704)	(7,857)
Subtotal	2,564	2,649	7,716	7,866
Measure of segment profitability (pretax)	2,564	2,649	7,716	7,866
Operating Segments [Member] | Investment [Member]
Revenue:
Net investment income	221,627	213,872	658,279	634,930
Total revenue	221,627	213,872	658,279	634,930
Expenses:
Required interest on reserves	192,854	184,912	572,201	547,443
Required interest on DAC	(55,188)	(52,946)	(163,802)	(157,096)
Interest expense	22,433	20,970	66,466	62,825
Total benefits and expenses	160,099	152,936	474,865	453,172
Subtotal	61,528	60,936	183,414	181,758
Measure of segment profitability (pretax)	61,528	60,936	183,414	181,758
Operating Segments [Member] | Other & Corporate [Member]
Revenue:
Other income	416	361	1,180	1,239
Total revenue	416	361	1,180	1,239
Expenses:
Insurance administrative expense	55,812	52,426	166,560	155,751
Parent expense	2,667	2,330	7,806	7,228
Stock-based compensation expense	10,412	8,263	29,969	24,809
Total benefits and expenses	68,891	63,019	204,335	187,788
Subtotal	(68,475)	(62,658)	(203,155)	(186,549)
Measure of segment profitability (pretax)	(68,475)	(62,658)	(203,155)	(186,549)
Adjustments [Member]
Revenue:
Other income	(23)	(30)	(76)	(99)
Total revenue	(23)	(30)	(76)	(99)
Expenses:
Policy benefits				2,094
Commissions, premium taxes, and non-deferred acquisition costs	(23)	1,362	(76)	1,293
Insurance administrative expense	3,590		3,590
Parent expense	1,578		1,578
Total benefits and expenses	5,145	1,362	5,092	3,387
Subtotal	(5,168)	(1,392)	(5,168)	(3,486)
Non-operating items	5,168	1,392	5,168	3,486
Measure of segment profitability (pretax)	$ 0	$ 0	$ 0	$ 0