Business Segments - Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenue:
Premium	$ 819,217	$ 783,411	$ 2,456,462	$ 2,349,126
Net investment income	213,872	202,720	634,930	601,415
Other income	331	160	1,140	963
Total revenue	1,033,420	986,291	3,092,532	2,951,504
Expenses:
Policy benefits	551,219	532,152	1,665,410	1,588,610
Required interest on reserves	0	0	0	0
Required interest on DAC	0	0	0	0
Amortization of acquisition costs	122,334	116,821	370,363	352,872
Commissions, premium taxes, and non-deferred acquisition costs	67,863	61,153	198,011	185,609
Insurance administrative expense	52,426	49,505	155,751	146,386
Parent expense	2,330	1,955	7,228	6,360
Stock compensation expense	8,263	6,345	24,809	20,334
Interest expense	20,970	20,381	62,825	62,860
Total benefits and expenses	825,405	788,312	2,484,397	2,363,031
Subtotal	208,015	197,979	608,135	588,473
Non-operating items	1,392	257	3,486	257
Measure of segment profitability (pretax)	209,407	198,236	611,621	588,730
Deduct applicable income taxes	(63,342)	(58,422)	(184,703)	(178,842)
Segment profits after tax	146,065	139,814	426,918	409,888
Add back income taxes applicable to segment profitability	63,342	58,422	184,703	178,842
Add (deduct) realized investment gains (losses)	12,595	3,482	6,142	7,780
Add (deduct) non-operating fees		(257)		(257)
Add (deduct) administrative settlements			(2,094)
Add (deduct) guaranty fund assessments	(1,392)		(1,392)
Income before income taxes	220,610	201,461	614,277	596,253
Operating Segments [Member] | Life [Member]
Revenue:
Premium	576,223	546,415	1,725,896	1,639,156
Total revenue	576,223	546,415	1,725,896	1,639,156
Expenses:
Policy benefits	386,445	369,546	1,166,289	1,101,748
Required interest on reserves	(152,862)	(145,295)	(452,339)	(430,931)
Required interest on DAC	46,893	44,950	139,042	133,628
Amortization of acquisition costs	98,268	93,496	296,646	281,698
Commissions, premium taxes, and non-deferred acquisition costs	44,748	40,577	132,094	121,968
Total benefits and expenses	423,492	403,274	1,281,732	1,208,111
Subtotal	152,731	143,141	444,164	431,045
Measure of segment profitability (pretax)	152,731	143,141	444,164	431,045
Operating Segments [Member] | Health [Member]
Revenue:
Premium	242,991	236,987	730,557	709,936
Total revenue	242,991	236,987	730,557	709,936
Expenses:
Policy benefits	155,774	153,351	470,104	459,387
Required interest on reserves	(19,617)	(18,476)	(57,859)	(54,803)
Required interest on DAC	5,881	5,789	17,530	17,297
Amortization of acquisition costs	23,458	22,643	71,801	67,110
Commissions, premium taxes, and non-deferred acquisition costs	21,746	20,604	64,599	63,733
Total benefits and expenses	187,242	183,911	566,175	552,724
Subtotal	55,749	53,076	164,382	157,212
Measure of segment profitability (pretax)	55,749	53,076	164,382	157,212
Operating Segments [Member] | Annuity [Member]
Revenue:
Premium	3	9	9	34
Total revenue	3	9	9	34
Expenses:
Policy benefits	9,000	9,255	26,923	27,475
Required interest on reserves	(12,433)	(12,761)	(37,245)	(38,359)
Required interest on DAC	172	192	524	621
Amortization of acquisition costs	608	682	1,916	4,064
Commissions, premium taxes, and non-deferred acquisition costs	7	11	25	31
Total benefits and expenses	(2,646)	(2,621)	(7,857)	(6,168)
Subtotal	2,649	2,630	7,866	6,202
Measure of segment profitability (pretax)	2,649	2,630	7,866	6,202
Operating Segments [Member] | Investment [Member]
Revenue:
Net investment income	213,872	202,720	634,930	601,415
Total revenue	213,872	202,720	634,930	601,415
Expenses:
Required interest on reserves	184,912	176,532	547,443	524,093
Required interest on DAC	(52,946)	(50,931)	(157,096)	(151,546)
Interest expense	20,970	20,381	62,825	62,860
Total benefits and expenses	152,936	145,982	453,172	435,407
Subtotal	60,936	56,738	181,758	166,008
Measure of segment profitability (pretax)	60,936	56,738	181,758	166,008
Operating Segments [Member] | Other & Corporate [Member]
Revenue:
Other income	361	199	1,239	1,086
Total revenue	361	199	1,239	1,086
Expenses:
Insurance administrative expense	52,426	49,248	155,751	146,129
Parent expense	2,330	1,955	7,228	6,360
Stock compensation expense	8,263	6,345	24,809	20,334
Total benefits and expenses	63,019	57,548	187,788	172,823
Subtotal	(62,658)	(57,349)	(186,549)	(171,737)
Measure of segment profitability (pretax)	(62,658)	(57,349)	(186,549)	(171,737)
Adjustments [Member]
Revenue:
Other income	(30)	(39)	(99)	(123)
Total revenue	(30)	(39)	(99)	(123)
Expenses:
Policy benefits			2,094
Commissions, premium taxes, and non-deferred acquisition costs	1,362	(39)	1,293	(123)
Insurance administrative expense		257		257
Total benefits and expenses	1,362	218	3,387	134
Subtotal	(1,392)	(257)	(3,486)	(257)
Non-operating items	1,392	257	3,486	257
Measure of segment profitability (pretax)	$ 0	$ 0	$ 0	$ 0