Schedule II. Condensed Financial Information of Registrant (Condensed Statement of Cash Flows) (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Net income	$ 187,061	$ 201,818	$ 186,566	$ 185,345	$ 164,717	$ 178,724	$ 184,425	$ 173,600	$ 760,790	$ 701,466	$ 1,454,494
Other, net									62,194	4,463	67,933
Cash provided from (used for) operating activities									1,363,874	1,277,647	1,429,058
Cash provided from (used for) investing activities:
Net decrease (increase) in short-term investments									34,003	63,783	(55,031)
Additions to properties									(42,203)	(45,092)	(20,285)
Net Cash Provided by (Used in) Investing Activities, Total									(809,311)	(896,223)	(926,824)
Cash provided from (used for) financing activities:
Repayment of debt									(6,875)	(327,762)	(126,875)
Proceeds from issuance of debt									0	550,000	125,000
Payment for debt issuance costs									0	(6,969)	(1,661)
Net issuance (repayment) of commercial paper									(11,610)	(22,719)	61,092
Issuance of stock									83,163	36,091	61,215
Acquisitions of treasury stock									(459,569)	(421,749)	(412,989)
Payment of dividends									(74,188)	(71,421)	(68,831)
Cash provided from (used for) financing activities									(590,508)	(391,520)	(453,981)
Net increase (decrease) in cash									(45,093)	2,463	42,400
Cash at beginning of year				121,026				118,563	121,026	118,563	76,163
Cash at end of year	75,933				121,026				75,933	121,026	118,563
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net income									760,790	701,466	1,454,494
Equity in earnings of affiliates									(810,596)	(767,506)	(1,538,174)
Cash dividends from subsidiaries									479,988	448,142	453,904
Other, net									65,584	64,734	52,957
Cash provided from (used for) operating activities									495,766	446,836	423,181
Cash provided from (used for) investing activities:
Net decrease (increase) in short-term investments									(3,380)	5,603	(5,624)
Investment in subsidiaries									0	(140,000)	(31,000)
Additions to properties									(32)	(19,888)	(7,230)
Loaned money to affiliates									(501,764)	(584,000)	(180,000)
Repayments from affiliates									501,764	584,000	180,000
Net Cash Provided by (Used in) Investing Activities, Total									(3,412)	(154,285)	(43,854)
Cash provided from (used for) financing activities:
Repayment of debt									(6,875)	(327,762)	(126,875)
Proceeds from issuance of debt									0	550,000	125,000
Payment for debt issuance costs									0	(6,969)	(1,661)
Net issuance (repayment) of commercial paper									(11,610)	(22,719)	61,092
Issuance of stock									82,771	36,091	61,215
Acquisitions of treasury stock									(459,569)	(421,749)	(412,989)
Borrowed money from affiliate									277,000	197,690	278,500
Repayments to affiliates									(276,500)	(202,690)	(270,500)
Payment of dividends									(97,458)	(94,691)	(92,101)
Cash provided from (used for) financing activities									(492,241)	(292,799)	(378,319)
Net increase (decrease) in cash									113	(248)	1,008
Cash at beginning of year				$ 760				$ 1,008	760	1,008	0
Cash at end of year	$ 873				$ 760				$ 873	$ 760	$ 1,008