Business Segments - Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Revenue:
Premium	$ 816,614	$ 785,855	$ 1,637,245	$ 1,565,715
Net investment income	212,776	201,642	421,058	398,695
Other income	393	382	809	803
Total revenue	1,029,783	987,879	2,059,112	1,965,213
Expenses:
Policy benefits	556,415	531,485	1,114,191	1,056,458
Required interest on reserves	0	0	0	0
Required interest on DAC	0	0	0	0
Amortization of acquisition costs	122,121	117,245	248,029	236,051
Commissions, premium taxes, and non-deferred acquisition costs	65,032	62,854	130,148	124,456
Insurance administrative expense	51,412	48,413	103,325	96,881
Parent expense	2,665	2,379	4,898	4,405
Stock compensation expense	8,351	7,054	16,546	13,989
Interest expense	21,156	23,110	41,855	42,479
Total benefits and expenses	827,152	792,540	1,658,992	1,574,719
Subtotal	202,631	195,339	400,120	390,494
Non-operating items	2,094	0	2,094	0
Measure of segment profitability (pretax)	204,725	195,339	402,214	390,494
Deduct applicable income taxes	(62,543)	(58,649)	(121,361)	(120,420)
Segment profits after tax	142,182	136,690	280,853	270,074
Add back income taxes applicable to segment profitability	62,543	58,649	121,361	120,420
Add (deduct) realized investment gains (losses)	(705)	4,005	(6,453)	4,298
Add (deduct) administrative settlements	(2,094)		(2,094)
Pretax income from continuing operations per Condensed Consolidated Statements of Operations	201,926	199,344	393,667	394,792
Operating Segments [Member] | Life [Member]
Revenue:
Premium	573,836	548,590	1,149,673	1,092,741
Total revenue	573,836	548,590	1,149,673	1,092,741
Expenses:
Policy benefits	388,765	369,342	779,844	732,202
Required interest on reserves	(150,652)	(143,625)	(299,477)	(285,636)
Required interest on DAC	46,213	44,476	92,149	88,678
Amortization of acquisition costs	98,473	93,663	198,378	188,202
Commissions, premium taxes, and non-deferred acquisition costs	43,708	41,130	87,346	81,391
Total benefits and expenses	426,507	404,986	858,240	804,837
Subtotal	147,329	143,604	291,433	287,904
Measure of segment profitability (pretax)	147,329	143,604	291,433	287,904
Operating Segments [Member] | Health [Member]
Revenue:
Premium	242,775	237,252	487,566	472,949
Total revenue	242,775	237,252	487,566	472,949
Expenses:
Policy benefits	156,579	153,261	314,330	306,036
Required interest on reserves	(19,267)	(18,251)	(38,242)	(36,327)
Required interest on DAC	5,840	5,766	11,649	11,508
Amortization of acquisition costs	23,016	22,102	48,343	44,467
Commissions, premium taxes, and non-deferred acquisition costs	21,351	21,753	42,853	43,129
Total benefits and expenses	187,519	184,631	378,933	368,813
Subtotal	55,256	52,621	108,633	104,136
Measure of segment profitability (pretax)	55,256	52,621	108,633	104,136
Operating Segments [Member] | Annuity [Member]
Revenue:
Premium	3	13	6	25
Total revenue	3	13	6	25
Expenses:
Policy benefits	8,977	8,882	17,923	18,220
Required interest on reserves	(12,394)	(12,506)	(24,812)	(25,598)
Required interest on DAC	174	205	352	429
Amortization of acquisition costs	632	1,480	1,308	3,382
Commissions, premium taxes, and non-deferred acquisition costs	7	11	18	20
Total benefits and expenses	(2,604)	(1,928)	(5,211)	(3,547)
Subtotal	2,607	1,941	5,217	3,572
Measure of segment profitability (pretax)	2,607	1,941	5,217	3,572
Operating Segments [Member] | Investment [Member]
Revenue:
Net investment income	212,776	201,642	421,058	398,695
Total revenue	212,776	201,642	421,058	398,695
Expenses:
Required interest on reserves	182,313	174,382	362,531	347,561
Required interest on DAC	(52,227)	(50,447)	(104,150)	(100,615)
Interest expense	21,156	23,110	41,855	42,479
Total benefits and expenses	151,242	147,045	300,236	289,425
Subtotal	61,534	54,597	120,822	109,270
Measure of segment profitability (pretax)	61,534	54,597	120,822	109,270
Operating Segments [Member] | Other & Corporate [Member]
Revenue:
Other income	427	422	878	887
Total revenue	427	422	878	887
Expenses:
Insurance administrative expense	51,412	48,413	103,325	96,881
Parent expense	2,665	2,379	4,898	4,405
Stock compensation expense	8,351	7,054	16,546	13,989
Total benefits and expenses	62,428	57,846	124,769	115,275
Subtotal	(62,001)	(57,424)	(123,891)	(114,388)
Measure of segment profitability (pretax)	(62,001)	(57,424)	(123,891)	(114,388)
Adjustments [Member]
Revenue:
Other income	(34)	(40)	(69)	(84)
Total revenue	(34)	(40)	(69)	(84)
Expenses:
Policy benefits	2,094		2,094
Commissions, premium taxes, and non-deferred acquisition costs	(34)	(40)	(69)	(84)
Total benefits and expenses	2,060	(40)	2,025	(84)
Subtotal	(2,094)	0	(2,094)	0
Non-operating items	2,094	0	2,094	0
Measure of segment profitability (pretax)	$ 0	$ 0	$ 0	$ 0