Business Segments - Analysis of Profitability by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Pretax operating income	$ 209,407	$ 198,236	$ 611,621	$ 588,730
Applicable taxes	(63,342)	(58,422)	(184,703)	(178,842)
Segment profits after tax	146,065	139,814	426,918	409,888
Discontinued operations (after tax)	(12)	9,959	(3,739)	(447)
After-tax total, after discontinued operations	146,053	149,773	423,179	409,441
Reconciling items, net of tax:
Realized gains (losses)—investments (after tax)	8,186	2,263	6,235	5,057
Administrative settlements (after tax)	0	0	(1,361)	0
Guaranty fund assessments (after tax)	(905)	0	(905)	0
Non-operating fees (after tax)	0	(167)	0	(167)
Net income	153,334	151,869	427,148	414,331
Operating Segments [Member] | Life insurance underwriting margin [Member]
Segment Reporting Information [Line Items]
Pretax operating income	152,731	143,141	444,164	431,045
Operating Segments [Member] | Health insurance underwriting margin [Member]
Segment Reporting Information [Line Items]
Pretax operating income	55,749	53,076	164,382	157,212
Operating Segments [Member] | Annuity underwriting margin [Member]
Segment Reporting Information [Line Items]
Pretax operating income	2,649	2,630	7,866	6,202
Operating Segments [Member] | Excessive investment income [Member]
Segment Reporting Information [Line Items]
Pretax operating income	60,936	56,738	181,758	166,008
Operating Segments [Member] | Other and corporate [Member]
Segment Reporting Information [Line Items]
Pretax operating income	(62,658)	(57,349)	(186,549)	(171,737)
Operating Segments [Member] | Other and corporate [Member] | Other income [Member]
Segment Reporting Information [Line Items]
Pretax operating income	361	199	1,239	1,086
Operating Segments [Member] | Other and corporate [Member] | Administrative expense [Member]
Segment Reporting Information [Line Items]
Pretax operating income	(52,426)	(49,248)	(155,751)	(146,129)
Operating Segments [Member] | Other and corporate [Member] | Corporate and adjustments [Member]
Segment Reporting Information [Line Items]
Pretax operating income	$ (10,593)	$ (8,300)	$ (32,037)	$ (26,694)