Debt - Selected Information about Debt Issues (Detail) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Par Value	$ 1,690,612,000
Less current maturity of term loan, par value	0
Total short-term debt, par value	255,000,000
Total debt, par value	1,945,612,000
Unamortized discount & issuance costs, noncurrent	(22,726,000)
Unamortized discount & issuance costs, current	(82,000)
Total debt, unamortized discount & issuance costs,	(22,808,000)
Book Value	1,667,886,000	$ 1,358,363,000
Less current maturity of term loan, book value	0	9,375,000
Total long-term debt, book value	1,667,886,000	1,348,988,000
Total short-term debt, book value	254,918,000	298,738,000
Total debt, book value	1,922,804,000	1,647,726,000
Fair Value	1,871,754,000
Less current maturity of term loan, fair value	0
Total long-term debt, fair value	1,871,754,000	1,473,364,000
Total short-term debt, fair value	254,918,000
Total debt, fair value	2,126,672,000
Commercial Paper
Debt Instrument [Line Items]
Total short-term debt, par value	255,000,000
Unamortized discount & issuance costs, current	(82,000)
Total short-term debt, book value	254,918,000	289,363,000
Total short-term debt, fair value	$ 254,918,000
Notes, Due 5/15/23
Debt Instrument [Line Items]
Coupon Rate	7.875%
Par Value	$ 165,612,000
Unamortized discount & issuance costs, noncurrent	(658,000)
Book Value	164,954,000	164,713,000
Fair Value	$ 192,020,000
Senior Notes Due 2022
Debt Instrument [Line Items]
Coupon Rate	3.80%
Par Value	$ 150,000,000
Unamortized discount & issuance costs, noncurrent	(586,000)
Book Value	149,414,000	149,089,000
Fair Value	158,081,000
Senior Notes Due 2022 | Consolidation eliminations | Subsidiaries
Debt Instrument [Line Items]
Par Value	150,000,000
Book Value	$ 150,000,000
Senior Notes, Due 2028
Debt Instrument [Line Items]
Coupon Rate	4.55%
Par Value	$ 550,000,000
Unamortized discount & issuance costs, noncurrent	(5,672,000)
Book Value	544,328,000	543,735,000
Fair Value	$ 659,599,000
Senior Notes, Due 2030
Debt Instrument [Line Items]
Coupon Rate	2.15%
Par Value	$ 400,000,000
Unamortized discount & issuance costs, noncurrent	(4,843,000)
Book Value	395,157,000	0
Fair Value	$ 405,384,000
Junior Subordinated Debentures Due 2056
Debt Instrument [Line Items]
Coupon Rate	6.125%
Par Value	$ 300,000,000
Unamortized discount & issuance costs, noncurrent	(9,348,000)
Book Value	290,652,000	290,584,000
Fair Value	$ 325,800,000
Junior Subordinated Debentures Due 2057
Debt Instrument [Line Items]
Coupon Rate	5.275%
Par Value	$ 125,000,000
Unamortized discount & issuance costs, noncurrent	(1,619,000)
Book Value	123,381,000	123,367,000
Fair Value	130,870,000
Term loan
Debt Instrument [Line Items]
Par Value	0
Less current maturity of term loan, par value	0
Unamortized discount & issuance costs, noncurrent	0
Book Value	0	86,875,000
Less current maturity of term loan, book value	0	$ 9,375,000
Fair Value	0
Less current maturity of term loan, fair value	$ 0