Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Statement of Cash Flows [Abstract]
Cash provided from operating activities	$ 616,520	$ 739,056
Investments sold or matured:
Fixed maturities available for sale—matured, called, and repaid	179,793	216,170
Other long-term investments	23	3,046
Total long-term investments sold or matured	179,816	219,216
Acquisition of investments:
Fixed maturities—available for sale	(540,442)	(676,648)
Other long-term investments	(64,444)	(1,775)
Total investments acquired	(604,886)	(678,423)
Net (increase) decrease in policy loans	(8,279)	(8,089)
Net (increase) decrease in short-term investments	58,527	(22,347)
Additions to property and equipment	(18,735)	(8,080)
Sale of other assets	17	18
Investment in low-income housing interests	(15,701)	(8,875)
Cash provided from (used for) investing activities	(409,241)	(506,580)
Cash provided from (used for) financing activities:
Issuance of common stock	21,352	27,970
Cash dividends paid to shareholders	(35,406)	(34,093)
Repayment of debt	(21,875)	(625)
Net borrowing (repayment) of commercial paper	50,213	40,546
Acquisition of treasury stock	(206,589)	(203,756)
Net receipts (payments) from deposit-type product	(58,662)	(44,294)
Cash provided from (used for) financing activities	(250,967)	(214,252)
Effect of foreign exchange rate changes on cash	7,355	3,265
Net increase (decrease) in cash	(36,333)	21,489
Cash at beginning of year	118,563	76,163
Cash at end of period	$ 82,230	$ 97,652