Schedule II. Condensed Financial Information of Registrant (Condensed Statement of Cash Flows) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Condensed Financial Statements, Captions [Line Items]
Net income	$ 274,802	$ 257,083	$ 215,260	$ 223,610	$ 242,343	$ 190,586	$ 223,973	$ 237,484	$ 970,755	$ 894,386	$ 1,031,114
Other, net									(24,799)	44,480	105,337
Cash provided from (used for) operating activities									1,482,425	1,422,194	1,437,680
Cash provided from (used for) investing activities:
Net decrease (increase) in short-term investments									32,381	(44,976)	38,637
Loaned money to affiliates									(49,553)	(27,929)	(38,244)
Cash provided from (used for) investing activities									(926,149)	(943,015)	(913,370)
Cash provided from (used for) financing activities:
Repayment of debt									(165,612)	(150,000)	(300,000)
Payment for debt issuance costs									(757)	(5,272)	(7,687)
Net borrowing (repayment) of commercial paper									32,961	(46,289)	74,974
Issuance of stock									114,080	106,592	69,826
Acquisitions of treasury stock									(511,100)	(454,638)	(541,435)
Payment of dividends									(84,116)	(80,547)	(80,043)
Cash provided from (used for) financing activities									(541,487)	(492,453)	(523,603)
Net increase (decrease) in cash									10,597	396	(2,684)
Cash at end of year	103,156				92,559				103,156	92,559	92,163
Cash at beginning of year				92,559				92,163	92,559	92,163	94,847
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net income									970,755	894,386	1,031,114
Equity in earnings of affiliates									(1,024,323)	(962,985)	(1,074,923)
Cash dividends from subsidiaries									459,535	407,042	478,535
Other, net									33,846	26,444	58,617
Cash provided from (used for) operating activities									439,813	364,887	493,343
Cash provided from (used for) investing activities:
Net decrease (increase) in short-term investments									13,996	(15,001)	19,300
Investment in subsidiaries									0	(10,010)	(159,924)
Other long-term investments									(3,950)	(2,000)	(2,500)
Loaned money to affiliates									(479,629)	(846,002)	(1,049,932)
Repayments from affiliates									505,929	886,002	1,200,932
Additions to properties									(7,400)	0	0
Cash provided from (used for) investing activities									28,946	12,989	7,876
Cash provided from (used for) financing activities:
Repayment of debt									(165,612)	(300,000)	(300,000)
Proceeds from issuance of debt									170,000	400,000	325,000
Payment for debt issuance costs									(757)	(5,272)	(7,687)
Net borrowing (repayment) of commercial paper									32,961	(46,289)	74,974
Issuance of stock									114,080	111,970	69,826
Acquisitions of treasury stock									(511,100)	(454,638)	(541,435)
Borrowed money from affiliate									290,500	22,400	32,000
Repayments to affiliates									(290,500)	(22,400)	(32,000)
Payment of dividends									(107,386)	(103,817)	(103,313)
Cash provided from (used for) financing activities									(467,814)	(398,046)	(482,635)
Net increase (decrease) in cash									945	(20,170)	18,584
Cash at end of year	$ 1,003				$ 58				1,003	58	20,228
Cash at beginning of year				$ 58				$ 20,228	$ 58	$ 20,228	$ 1,644