Schedule II. Condensed Financial Information of Registrant (Condensed Statement of Cash Flows) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net income	$ 1,027,346	$ 153,334	$ 140,273	$ 133,541	$ 135,448	$ 151,869	$ 138,429	$ 124,033	$ 1,454,494	$ 549,779	$ 527,100
Other, net									67,933	20,079	13,985
Cash provided from (used for) operating activities									1,429,058	1,398,708	1,120,050
Cash provided from (used for) investing activities:
Disposition of investments									559,623	578,004	606,690
Net decrease (increase) in short-term investments									(55,031)	(17,274)	(38,884)
Additions to properties									(20,285)	(25,162)	(36,957)
Cash provided from (used for) investing activities									(926,824)	(1,062,436)	(633,350)
Cash provided from (used for) financing activities:
Repayment of debt									(126,875)	(250,000)	0
Proceeds from issuance of debt									125,000	400,000	0
Payment for debt issuance costs									(1,661)	(9,638)	0
Net issuance (repayment) of commercial paper									61,092	22,224	1,978
Issuance of stock									61,215	61,329	35,958
Acquisitions of treasury stock									(412,989)	(404,784)	(418,526)
Excess tax benefit from stock option exercises									0	0	17,577
Payment of dividends									(68,831)	(66,931)	(66,899)
Cash provided from (used for) financing activities									(453,981)	(319,791)	(525,705)
Increase (decrease) in cash									42,400	14,780	(4,636)
Cash at beginning of year				76,163				61,383	76,163	61,383	66,019
Cash at end of year	118,563				76,163				118,563	76,163	61,383
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net income									1,454,494	549,779	527,100
Equity in earnings of affiliates									(1,538,174)	(586,126)	(568,176)
Cash dividends from subsidiaries									453,904	437,566	466,416
Other, net									52,957	(6,718)	20,371
Cash provided from (used for) operating activities									423,181	394,501	445,711
Cash provided from (used for) investing activities:
Net decrease (increase) in short-term investments									(5,624)	(3,466)	17,338
Investment in subsidiaries									(31,000)	(35,000)	(2)
Additions to properties									(7,230)	(21,965)	(468)
Loaned money to affiliates									(180,000)	(363,056)	(282,508)
Repayments from affiliates									180,000	318,056	282,508
Cash provided from (used for) investing activities									(43,854)	(105,431)	16,868
Cash provided from (used for) financing activities:
Repayment of debt									(126,875)	(250,000)	0
Proceeds from issuance of debt									125,000	400,000	0
Payment for debt issuance costs									(1,661)	(9,638)	0
Net issuance (repayment) of commercial paper									61,092	22,224	1,978
Issuance of stock									61,215	61,329	35,958
Acquisitions of treasury stock									(412,989)	(404,784)	(418,526)
Borrowed money from affiliate									278,500	60,000	15,000
Repayments to affiliates									(270,500)	(78,000)	(15,000)
Excess tax benefit from stock option exercises									0	0	8,180
Payment of dividends									(92,101)	(90,201)	(90,169)
Cash provided from (used for) financing activities									(378,319)	(289,070)	(462,579)
Increase (decrease) in cash									1,008	0	0
Cash at beginning of year				$ 0				$ 0	0	0	0
Cash at end of year	$ 1,008				$ 0				$ 1,008	$ 0	$ 0