Business Segments - Reconciliation of Segment Operating Information to Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2016		Jun. 30, 2015		Jun. 30, 2016		Jun. 30, 2015
Revenue:
Premium	$ 785,855	[1]	$ 752,484	[2]	$ 1,565,715	[1]	$ 1,494,540	[2]
Net investment income	201,642	[1]	194,823	[2]	398,695	[1]	386,419	[2]
Other income	382	[1]	691	[2]	803	[1]	1,360	[2]
Total revenue	987,879		947,998		1,965,213		1,882,319
Expenses:
Policy benefits	531,485	[1]	508,316	[2]	1,056,458	[1]	1,006,091	[2]
Required interest on reserves	0		0		0		0
Required interest on DAC	0		0		0		0
Amortization of acquisition costs	117,245	[1]	111,738	[2]	236,051	[1]	222,398	[2]
Commissions, premium taxes, and non-deferred acquisition costs	62,854	[1]	59,132	[2]	124,456	[1]	116,237	[2]
Insurance administrative expense	48,413		45,474		96,881		91,425
Parent expense	2,379		2,312		4,405		4,485
Stock compensation expense	7,054		7,802		13,989		15,041
Interest expense	23,110	[1]	19,114	[2]	42,479	[1]	38,174	[2]
Total benefits and expenses	792,540	[1]	753,888	[2]	1,574,719	[1]	1,493,851	[2]
Subtotal	195,339		194,110		390,494		388,468
Nonoperating items	0		0		0		0
Measure of segment profitability (pretax)	195,339		194,110		390,494		388,468
Deduct applicable income taxes	(58,649)		(63,282)		(120,420)		(126,939)
Segment profits after tax	136,690		130,828		270,074		261,529
Add (deduct) realized investment gains (losses)	4,005		2,613		4,298		2,732
Income before income taxes	199,344	[1]	196,723	[2]	394,792	[1]	391,200	[2]
Operating Segments [Member] | Life [Member]
Revenue:
Premium	548,590		520,038		1,092,741		1,033,380
Total revenue	548,590		520,038		1,092,741		1,033,380
Expenses:
Policy benefits	369,342		347,364		732,202		687,065
Required interest on reserves	(143,625)		(137,430)		(285,636)		(273,615)
Required interest on DAC	44,476		43,139		88,678		85,985
Amortization of acquisition costs	93,663		88,737		188,202		177,265
Commissions, premium taxes, and non-deferred acquisition costs	41,130		38,851		81,391		75,900
Total benefits and expenses	404,986		380,661		804,837		752,600
Subtotal	143,604		139,377		287,904		280,780
Measure of segment profitability (pretax)	143,604		139,377		287,904		280,780
Operating Segments [Member] | Health [Member]
Revenue:
Premium	237,252		232,409		472,949		461,082
Total revenue	237,252		232,409		472,949		461,082
Expenses:
Policy benefits	153,261		151,198		306,036		299,227
Required interest on reserves	(18,251)		(17,151)		(36,327)		(34,034)
Required interest on DAC	5,766		5,690		11,508		11,358
Amortization of acquisition costs	22,102		20,740		44,467		40,924
Commissions, premium taxes, and non-deferred acquisition costs	21,753		20,320		43,129		40,418
Total benefits and expenses	184,631		180,797		368,813		357,893
Subtotal	52,621		51,612		104,136		103,189
Measure of segment profitability (pretax)	52,621		51,612		104,136		103,189
Operating Segments [Member] | Annuity [Member]
Revenue:
Premium	13		37		25		78
Total revenue	13		37		25		78
Expenses:
Policy benefits	8,882		9,754		18,220		19,799
Required interest on reserves	(12,506)		(13,387)		(25,598)		(26,756)
Required interest on DAC	205		294		429		607
Amortization of acquisition costs	1,480		2,261		3,382		4,209
Commissions, premium taxes, and non-deferred acquisition costs	11		12		20		23
Total benefits and expenses	(1,928)		(1,066)		(3,547)		(2,118)
Subtotal	1,941		1,103		3,572		2,196
Measure of segment profitability (pretax)	1,941		1,103		3,572		2,196
Operating Segments [Member] | Investment [Member]
Revenue:
Net investment income	201,642		194,823		398,695		386,419
Total revenue	201,642		194,823		398,695		386,419
Expenses:
Required interest on reserves	174,382		167,968		347,561		334,405
Required interest on DAC	(50,447)		(49,123)		(100,615)		(97,950)
Interest expense	23,110		19,114		42,479		38,174
Total benefits and expenses	147,045		137,959		289,425		274,629
Subtotal	54,597		56,864		109,270		111,790
Measure of segment profitability (pretax)	54,597		56,864		109,270		111,790
Operating Segments [Member] | Other & Corporate [Member]
Revenue:
Other income	422		742		887		1,464
Total revenue	422		742		887		1,464
Expenses:
Insurance administrative expense	48,413		45,474		96,881		91,425
Parent expense	2,379		2,312		4,405		4,485
Stock compensation expense	7,054		7,802		13,989		15,041
Total benefits and expenses	57,846		55,588		115,275		110,951
Subtotal	(57,424)		(54,846)		(114,388)		(109,487)
Measure of segment profitability (pretax)	(57,424)		(54,846)		(114,388)		(109,487)
Adjustments [Member]
Revenue:
Other income	(40)		(51)		(84)		(104)
Total revenue	(40)		(51)		(84)		(104)
Expenses:
Commissions, premium taxes, and non-deferred acquisition costs	(40)		(51)		(84)		(104)
Total benefits and expenses	(40)		(51)		(84)		(104)
Subtotal	0		0		0		0
Nonoperating items	0		0		0		0
Measure of segment profitability (pretax)	$ 0		$ 0		$ 0		$ 0

[1] Due to the adoption of ASU 2016-09, certain balances related to excess tax benefits from stock compensation were adjusted prospectively as described in Note 2—New Accounting Standards.

[2] Certain prior year balances were adjusted to give effect to discontinued operations as described in Note 5—Discontinued Operations.