Business Segments - Analysis of Profitability by Segment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Pretax operating income	$ 204,725	$ 195,339	$ 402,214	$ 390,494
Applicable taxes	(62,543)	(58,649)	(121,361)	(120,420)
Segment profits after tax	142,182	136,690	280,853	270,074
Discontinued operations (after tax)	(90)	(865)	(3,727)	(10,406)
Net operating income	142,092	135,825	277,126	259,668
Reconciling items, net of tax:
Realized gains (losses)—investments (after tax)	(458)	2,604	(1,951)	2,794
Administrative settlements (after tax)	(1,361)	0	(1,361)	0
Net income	140,273	138,429	273,814	262,462
Operating Segments [Member] | Life insurance underwriting margin [Member]
Segment Reporting Information [Line Items]
Pretax operating income	147,329	143,604	291,433	287,904
Operating Segments [Member] | Health insurance underwriting margin [Member]
Segment Reporting Information [Line Items]
Pretax operating income	55,256	52,621	108,633	104,136
Operating Segments [Member] | Annuity underwriting margin [Member]
Segment Reporting Information [Line Items]
Pretax operating income	2,607	1,941	5,217	3,572
Operating Segments [Member] | Excessive investment income [Member]
Segment Reporting Information [Line Items]
Pretax operating income	61,534	54,597	120,822	109,270
Operating Segments [Member] | Other and corporate [Member]
Segment Reporting Information [Line Items]
Pretax operating income	(62,001)	(57,424)	(123,891)	(114,388)
Operating Segments [Member] | Other and corporate [Member] | Other income [Member]
Segment Reporting Information [Line Items]
Pretax operating income	427	422	878	887
Operating Segments [Member] | Other and corporate [Member] | Administrative expense [Member]
Segment Reporting Information [Line Items]
Pretax operating income	(51,412)	(48,413)	(103,325)	(96,881)
Operating Segments [Member] | Other and corporate [Member] | Corporate and adjustments [Member]
Segment Reporting Information [Line Items]
Pretax operating income	$ (11,016)	$ (9,433)	$ (21,444)	$ (18,394)