Business Segments - Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue:
Premium	$ 853,979	$ 816,614	$ 1,704,085	$ 1,637,245
Net investment income	218,568	212,776	436,652	421,058
Other income	416	393	711	809
Total revenue	1,072,963	1,029,783	2,141,448	2,059,112
Expenses:
Policy benefits	568,377	556,415	1,138,266	1,114,191
Required interest on reserves	0	0	0	0
Required interest on DAC	0	0	0	0
Amortization of acquisition costs	129,077	122,121	258,697	248,029
Commissions, premium taxes, and non-deferred acquisition costs	69,427	65,032	139,066	130,148
Insurance administrative expense	55,276	51,412	110,748	103,325
Parent expense	2,847	2,665	5,139	4,898
Stock-based compensation expense	10,497	8,351	19,557	16,546
Interest expense	22,411	21,156	44,033	41,855
Total benefits and expenses	857,912	827,152	1,715,506	1,658,992
Subtotal	215,051	202,631	425,942	400,120
Non-operating items	0	2,094	0	2,094
Measure of segment profitability (pretax)	215,051	204,725	425,942	402,214
Deduct applicable income taxes	(39,991)	(62,543)	(78,712)	(121,361)
Net operating income	175,060	142,182	347,230	280,853
Add back income taxes applicable to segment profitability	39,991	62,543	78,712	121,361
Add (deduct) realized investment gains (losses)	11,813	(705)	13,764	(6,453)
Add (deduct) administrative settlements		(2,094)		(2,094)
Income before income taxes	226,864	201,926	439,706	393,667
Operating Segments [Member] | Life [Member]
Revenue:
Premium	602,534	573,836	1,200,837	1,149,673
Total revenue	602,534	573,836	1,200,837	1,149,673
Expenses:
Policy benefits	399,334	388,765	799,915	779,844
Required interest on reserves	(158,101)	(150,652)	(314,315)	(299,477)
Required interest on DAC	48,275	46,213	96,219	92,149
Amortization of acquisition costs	104,599	98,473	208,376	198,378
Commissions, premium taxes, and non-deferred acquisition costs	47,250	43,708	94,644	87,346
Total benefits and expenses	441,357	426,507	884,839	858,240
Subtotal	161,177	147,329	315,998	291,433
Measure of segment profitability (pretax)	161,177	147,329	315,998	291,433
Operating Segments [Member] | Health [Member]
Revenue:
Premium	251,440	242,775	503,238	487,566
Total revenue	251,440	242,775	503,238	487,566
Expenses:
Policy benefits	160,461	156,579	321,080	314,330
Required interest on reserves	(20,726)	(19,267)	(41,130)	(38,242)
Required interest on DAC	6,079	5,840	12,092	11,649
Amortization of acquisition costs	23,892	23,016	49,149	48,343
Commissions, premium taxes, and non-deferred acquisition costs	22,196	21,351	44,461	42,853
Total benefits and expenses	191,902	187,519	385,652	378,933
Subtotal	59,538	55,256	117,586	108,633
Measure of segment profitability (pretax)	59,538	55,256	117,586	108,633
Operating Segments [Member] | Annuity [Member]
Revenue:
Premium	5	3	10	6
Total revenue	5	3	10	6
Expenses:
Policy benefits	8,582	8,977	17,271	17,923
Required interest on reserves	(11,885)	(12,394)	(23,902)	(24,812)
Required interest on DAC	150	174	303	352
Amortization of acquisition costs	586	632	1,172	1,308
Commissions, premium taxes, and non-deferred acquisition costs	6	7	14	18
Total benefits and expenses	(2,561)	(2,604)	(5,142)	(5,211)
Subtotal	2,566	2,607	5,152	5,217
Measure of segment profitability (pretax)	2,566	2,607	5,152	5,217
Operating Segments [Member] | Investment [Member]
Revenue:
Net investment income	218,568	212,776	436,652	421,058
Total revenue	218,568	212,776	436,652	421,058
Expenses:
Required interest on reserves	190,712	182,313	379,347	362,531
Required interest on DAC	(54,504)	(52,227)	(108,614)	(104,150)
Interest expense	22,411	21,156	44,033	41,855
Total benefits and expenses	158,619	151,242	314,766	300,236
Subtotal	59,949	61,534	121,886	120,822
Measure of segment profitability (pretax)	59,949	61,534	121,886	120,822
Operating Segments [Member] | Other & Corporate [Member]
Revenue:
Other income	441	427	764	878
Total revenue	441	427	764	878
Expenses:
Insurance administrative expense	55,276	51,412	110,748	103,325
Parent expense	2,847	2,665	5,139	4,898
Stock-based compensation expense	10,497	8,351	19,557	16,546
Total benefits and expenses	68,620	62,428	135,444	124,769
Subtotal	(68,179)	(62,001)	(134,680)	(123,891)
Measure of segment profitability (pretax)	(68,179)	(62,001)	(134,680)	(123,891)
Adjustments [Member]
Revenue:
Other income	(25)	(34)	(53)	(69)
Total revenue	(25)	(34)	(53)	(69)
Expenses:
Policy benefits		2,094		2,094
Commissions, premium taxes, and non-deferred acquisition costs	(25)	(34)	(53)	(69)
Insurance administrative expense
Total benefits and expenses	(25)	2,060	(53)	2,025
Subtotal	0	(2,094)	0	(2,094)
Non-operating items	0	2,094	0	2,094
Measure of segment profitability (pretax)	$ 0	$ 0	$ 0	$ 0