Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 369,116	$ 231,092	$ 190,455
Adjustments to reconcile net income to net cash provided by operating activities:
Losses allocated to participating policyholders	(1,669)	(1,024)	(1,267)
(Accretion of discounts) amortization of premiums on investments, net	(3,943)	2,209	(12,213)
Net realized gains on investments	(47,035)	(75,472)	(73,331)
Net proceeds (purchases) of trading securities	491,154	107,770	(277,510)
Interest credited to contractholders	631,051	606,790	577,548
Depreciation and amortization	69,781	74,987	139,735
Deferral of acquisition costs	(107,353)	(93,621)	(64,709)
Deferred income taxes	(167,059)	53,481	21,682
Contingent consideration	0	(209)	(17,600)
Amortization of low-income housing partnerships	517	372	4,563
Other, net	(5,052)	(1,366)	(3,072)
Changes in assets and liabilities:
Policy benefit liabilities	(320,518)	(231,952)	(273,507)
Reinsurance recoverable	11,341	10,340	8,738
Investment income due and accrued	(11,764)	(4,392)	(4,385)
Other, net	32,406	(23,188)	10,213
Net cash provided by operating activities	940,973	655,817	225,340
Proceeds from sales, maturities, and redemptions of investments:
Fixed maturities, available-for-sale	5,366,470	6,229,209	5,470,124
Mortgage loans on real estate	426,421	389,663	594,497
Limited partnership interests, other corporation interests, and other investments	20,916	10,742	6,833
Purchases of investments:
Fixed maturities, available-for-sale	(6,442,306)	(7,840,166)	(6,468,699)
Mortgage loans on real estate	(854,459)	(694,127)	(448,924)
Limited partnership interests, other corporation interests, and other investments	(22,255)	(5,766)	(1,527)
Net change in short-term investments	(53,097)	(39,677)	(2,238)
Policy loans, net	(10,553)	5,527	98,143
Purchases of furniture, equipment, and software	(40,997)	(44,644)	(78,778)
Net cash used in investing activities	(1,609,860)	(1,989,239)	(830,569)
Cash flows from financing activities:
Contract deposits	3,000,182	3,546,320	2,527,039
Contract withdrawals	(2,240,275)	(2,098,286)	(1,782,571)
Change in due to/from parent and affiliates	(16,227)	(21,719)	(22,359)
Dividends paid	(145,301)	(125,691)	(139,533)
Capital contribution	76,429	0	0
Proceeds from issuance of common stock	8,515	19,750	60,803
Payments for and interest paid on financing element derivatives, net	(3,233)	(6,744)	(9,383)
Payment of contingent consideration	0	(14,400)	0
Net commercial paper borrowings	837	5,678	(5,218)
Change in book overdrafts	(12,332)	12,713	(1,651)
Employee taxes paid for withheld shares	(818)	(517)	(1,264)
Income tax benefit of stock option exercises	0	277	953
Net cash provided by financing activities	667,777	1,317,381	626,816
Net (decrease) increase in cash and cash equivalents	(1,110)	(16,041)	21,587
Cash and cash equivalents, beginning of year	18,321	34,362	12,775
Cash and cash equivalents, end of year	17,211	18,321	34,362
Net cash (paid) received during the year for:
Income taxes	(22,668)	(1,543)	(4,093)
Interest	(26,630)	(31,254)	(37,288)
Non-cash investing and financing transactions during the years:
Share-based compensation expense	(1,572)	(2,190)	(1,655)
Fair value of assets acquired in settlement of fixed maturity investments	$ 9,659	$ 0	$ 0