Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Statement of Cash Flows [Abstract]
Cash provided from operating activities	$ 937,289	$ 1,085,842
Investments sold or matured:
Fixed maturities available for sale—sold	12,162	52,951
Fixed maturities available for sale—matured, called, and repaid	266,736	306,132
Other long-term investments	47	3,523
Total investments sold or matured	278,945	362,606
Acquisition of investments:
Fixed maturities—available for sale	(727,296)	(1,042,705)
Other long-term investments	(74,396)	(16,775)
Total investments acquired	(801,692)	(1,059,480)
Net (increase) decrease in policy loans	(14,307)	(15,343)
Net (increase) decrease in short-term investments	(506,419)	6,556
Additions to property and equipment	(27,316)	(13,451)
Sale of other assets	17	18
Investment in low-income housing interests	(19,075)	(13,852)
Cash provided from (used for) investing activities	(1,089,847)	(732,946)
Cash provided from (used for) financing activities:
Issuance of common stock	35,065	40,891
Cash dividends paid to shareholders	(53,481)	(51,532)
Proceeds from issuance of debt	550,000	0
Payment for debt issuance costs	(6,797)	0
Repayment of debt	(23,125)	(1,250)
Net borrowing (repayment) of commercial paper	(7,196)	42,652
Acquisition of treasury stock	(298,314)	(301,448)
Net receipts (payments) from deposit-type product	(87,561)	(65,912)
Cash provided from (used for) financing activities	108,591	(336,599)
Effect of foreign exchange rate changes on cash	2,843	(3,998)
Net increase (decrease) in cash	(41,124)	12,299
Cash at beginning of year	118,563	76,163
Cash at end of period	$ 77,439	$ 88,462