Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Cash Flows [Abstract]
Cash provided from operating activities	$ 382,389	$ 460,756
Investments sold or matured:
Fixed maturities available for sale—matured, called, and repaid	74,019	108,899
Other long-term investments	0	3,071
Total long-term investments sold or matured	74,019	111,970
Acquisition of investments:
Fixed maturities—available for sale	(358,753)	(461,324)
Other long-term investments	(53,853)	0
Total investments acquired	(412,606)	(461,324)
Net (increase) decrease in policy loans	(2,854)	(2,599)
Net (increase) decrease in short-term investments	47,048	(48,950)
Additions to property and equipment	(11,966)	(3,978)
Sale of other assets	0	18
Investment in low-income housing interests	(7,518)	(5,875)
Cash provided from (used for) investing activities	(313,877)	(410,738)
Cash provided from (used for) financing activities:
Issuance of common stock	13,569	22,225
Cash dividends paid to shareholders	(17,194)	(16,503)
Repayment of debt	(625)	0
Net borrowing (repayment) of commercial paper	36,464	61,919
Acquisition of treasury stock	(109,954)	(113,719)
Net receipts (payments) from deposit-type product	(25,603)	(21,174)
Cash provided from (used for) financing activities	(103,343)	(67,252)
Effect of foreign exchange rate changes on cash	4,883	2,825
Net increase (decrease) in cash	(29,948)	(14,409)
Cash at beginning of year	118,563	76,163
Cash at end of period	$ 88,615	$ 61,754