Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Statement of Cash Flows [Abstract]
Cash provided from operating activities	$ 739,056	$ 604,935
Investments sold or matured:
Fixed maturities available for sale—sold	0	51,299
Fixed maturities available for sale—matured, called, and repaid	216,170	92,475
Other long-term investments	3,046	1,394
Total long-term investments sold or matured	219,216	145,168
Acquisition of investments:
Fixed maturities—available for sale	(676,648)	(651,267)
Other long-term investments	(1,775)	(21,762)
Total investments acquired	(678,423)	(673,029)
Net increase in policy loans	(8,089)	(9,093)
Net (increase) decrease in short-term investments	(22,347)	6,185
Net change in payable or receivable for securities	0	(711)
Additions to property and equipment	(8,080)	(6,740)
Sale of other assets	18	0
Investment in low-income housing interests	(8,875)	(9,260)
Cash provided from (used for) investing activities	(506,580)	(547,480)
Cash provided from (used for) financing activities:
Issuance of common stock	27,970	25,286
Cash dividends paid to shareholders	(34,093)	(33,478)
Proceeds from issuance of debt	0	400,000
Payment for debt issuance costs	0	(9,638)
Repayment of debt	(625)	(250,000)
Net borrowing (repayment) of commercial paper	40,546	45,010
Acquisition of treasury stock	(203,756)	(202,975)
Net receipts (payments) from deposit-type product	(44,294)	(38,193)
Cash provided from (used for) financing activities	(214,252)	(63,988)
Effect of foreign exchange rate changes on cash	3,265	(5,172)
Net increase (decrease) in cash	21,489	(11,705)
Cash at beginning of year	76,163	61,383
Cash at end of period	$ 97,652	$ 49,678