Business Segments - Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue:
Premium	$ 850,106	$ 820,631
Net investment income	218,084	208,282
Other income	295	416
Total revenue	1,068,485	1,029,329
Expenses:
Policy benefits	569,889	557,776
Required interest on reserves	0	0
Required interest on DAC	0	0
Amortization of acquisition costs	129,620	125,908
Commissions, premium taxes, and non-deferred acquisition costs	69,639	65,116
Insurance administrative expense	55,472	51,913
Parent expense	2,292	2,233
Stock-based compensation expense	9,060	8,195
Interest expense	21,622	20,699
Total benefits and expenses	857,594	831,840
Subtotal	210,891	197,489
Non-operating items	0	0
Measure of segment profitability (pretax)	210,891	197,489
Deduct applicable income taxes	(38,721)	(58,818)
Net operating income	172,170	138,671
Add back income taxes applicable to segment profitability	38,721	58,818
Add (deduct) realized investment gains (losses)	1,951	(5,748)
Income before income taxes	212,842	191,741
Operating Segments [Member] | Life [Member]
Revenue:
Premium	598,303	575,837
Total revenue	598,303	575,837
Expenses:
Policy benefits	400,581	391,079
Required interest on reserves	(156,214)	(148,825)
Required interest on DAC	47,944	45,936
Amortization of acquisition costs	103,777	99,905
Commissions, premium taxes, and non-deferred acquisition costs	47,394	43,638
Total benefits and expenses	443,482	431,733
Subtotal	154,821	144,104
Measure of segment profitability (pretax)	154,821	144,104
Operating Segments [Member] | Health [Member]
Revenue:
Premium	251,798	244,791
Total revenue	251,798	244,791
Expenses:
Policy benefits	160,619	157,751
Required interest on reserves	(20,404)	(18,975)
Required interest on DAC	6,013	5,809
Amortization of acquisition costs	25,257	25,327
Commissions, premium taxes, and non-deferred acquisition costs	22,265	21,502
Total benefits and expenses	193,750	191,414
Subtotal	58,048	53,377
Measure of segment profitability (pretax)	58,048	53,377
Operating Segments [Member] | Annuity [Member]
Revenue:
Premium	5	3
Total revenue	5	3
Expenses:
Policy benefits	8,689	8,946
Required interest on reserves	(12,017)	(12,418)
Required interest on DAC	153	178
Amortization of acquisition costs	586	676
Commissions, premium taxes, and non-deferred acquisition costs	8	11
Total benefits and expenses	(2,581)	(2,607)
Subtotal	2,586	2,610
Measure of segment profitability (pretax)	2,586	2,610
Operating Segments [Member] | Investment [Member]
Revenue:
Net investment income	218,084	208,282
Total revenue	218,084	208,282
Expenses:
Required interest on reserves	188,635	180,218
Required interest on DAC	(54,110)	(51,923)
Interest expense	21,622	20,699
Total benefits and expenses	156,147	148,994
Subtotal	61,937	59,288
Measure of segment profitability (pretax)	61,937	59,288
Operating Segments [Member] | Other & Corporate [Member]
Revenue:
Other income	323	451
Total revenue	323	451
Expenses:
Insurance administrative expense	55,472	51,913
Parent expense	2,292	2,233
Stock-based compensation expense	9,060	8,195
Total benefits and expenses	66,824	62,341
Subtotal	(66,501)	(61,890)
Measure of segment profitability (pretax)	(66,501)	(61,890)
Adjustments [Member]
Revenue:
Other income	(28)	(35)
Total revenue	(28)	(35)
Expenses:
Policy benefits
Commissions, premium taxes, and non-deferred acquisition costs	(28)	(35)
Insurance administrative expense
Total benefits and expenses	(28)	(35)
Subtotal	0	0
Non-operating items	0	0
Measure of segment profitability (pretax)	$ 0	$ 0