Long Term Debt - Senior Notes - Additional Information (Detail)				3 Months Ended		9 Months Ended
	Jan. 19, 2018 USD ($)	Jun. 14, 2012 USD ($) Covenant	Oct. 05, 2010 USD ($) Covenant	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
Debt Instrument [Line Items]
Repayment of borrowings						$ 347,000,000	$ 138,000,000
Proceeds from borrowings, before offering expenses						79,000,000	282,000,000
Interest expense related to amortization of debt issuance costs				$ 356,000	$ 710,000	$ 1,600,000	$ 2,000,000
Revolving Credit Facility
Debt Instrument [Line Items]
Repayment of borrowings	$ 239,000,000
4.97% Series A Senior Notes, Due October 5th 2020
Debt Instrument [Line Items]
Debt maturity date			Oct. 05, 2020
Long-term debt, aggregate principal amount			$ 300,000,000
Debt interest rate			4.97%
Semi-annual interest payment, first payment date						April 5
Semi-annual interest payment, second payment date						October 5
Notes issuance date			Oct. 05, 2010
Description of the prepayment terms						Notes are prepayable at the Company’s option, in whole or in part, provided that in the case of a partial prepayment, prepayment must be in an amount not less than 5% of the aggregate principal amount of the notes then outstanding, at any time and from time to time at 100% of the principal amount prepaid, plus accrued and unpaid interest to the prepayment date, plus a “make-whole” premium as specified in the note purchase agreements. The Company must offer to prepay the notes upon the occurrence of any change of control. In addition, the Company must offer to prepay the notes upon the occurrence of certain asset dispositions if the proceeds therefrom are not timely reinvested in productive assets. If any offer to prepay is accepted, the purchase price of each prepaid note is 100% of the principal amount thereof, plus accrued and unpaid interest thereon to the prepayment date.
Debt instrument, redemption percentage			100.00%
Description of the acceptance terms						If any offer to prepay is accepted, the purchase price of each prepaid note is 100% of the principal amount thereof, plus accrued and unpaid interest thereon to the prepayment date.
Acceptance terms, percent of principal before accrued and unpaid interest			100.00%
Note purchase agreement, financial covenant description						The Company must not permit its debt to capitalization ratio to exceed 50% at any time. The note purchase agreements generally define the debt to capitalization ratio as the ratio of (a) total borrowed money indebtedness to (b) the sum of such indebtedness plus consolidated net worth, with consolidated net worth determined as of the last day of the most recently ended fiscal quarter. The Company also must not permit its interest coverage ratio as of the last day of a fiscal quarter to be less than 2.50 to 1.00. The note purchase agreements generally define the interest coverage ratio as the ratio of EBITDA for the four prior fiscal quarters to interest charges for the same period. The Company was in compliance with these covenants at September 30, 2018.
Number of compliance covenants | Covenant			2
4.97% Series A Senior Notes, Due October 5th 2020 | Minimum
Debt Instrument [Line Items]
Debt instrument, prepayment percentage of aggregate principal amount			5.00%
Interest coverage ratio that the Company must exceed on the last day of the fiscal quarter			2.50%
4.97% Series A Senior Notes, Due October 5th 2020 | Maximum
Debt Instrument [Line Items]
Debt to capitalization ratio, percentage the Company must not exceed at any time			50.00%
4.27% Series B Senior Notes, Due June 14th 2022
Debt Instrument [Line Items]
Debt maturity date		Jun. 14, 2022
Long-term debt, aggregate principal amount		$ 300,000,000
Debt interest rate		4.27%
Semi-annual interest payment, first payment date						April 5
Semi-annual interest payment, second payment date						October 5
Notes issuance date		Jun. 14, 2012
Description of the prepayment terms						Notes are prepayable at the Company’s option, in whole or in part, provided that in the case of a partial prepayment, prepayment must be in an amount not less than 5% of the aggregate principal amount of the notes then outstanding, at any time and from time to time at 100% of the principal amount prepaid, plus accrued and unpaid interest to the prepayment date, plus a “make-whole” premium as specified in the note purchase agreements. The Company must offer to prepay the notes upon the occurrence of any change of control. In addition, the Company must offer to prepay the notes upon the occurrence of certain asset dispositions if the proceeds therefrom are not timely reinvested in productive assets. If any offer to prepay is accepted, the purchase price of each prepaid note is 100% of the principal amount thereof, plus accrued and unpaid interest thereon to the prepayment date.
Debt instrument, redemption percentage		100.00%
Description of the acceptance terms						If any offer to prepay is accepted, the purchase price of each prepaid note is 100% of the principal amount thereof, plus accrued and unpaid interest thereon to the prepayment date.
Acceptance terms, percent of principal before accrued and unpaid interest		100.00%
Note purchase agreement, financial covenant description						The Company must not permit its debt to capitalization ratio to exceed 50% at any time. The note purchase agreements generally define the debt to capitalization ratio as the ratio of (a) total borrowed money indebtedness to (b) the sum of such indebtedness plus consolidated net worth, with consolidated net worth determined as of the last day of the most recently ended fiscal quarter. The Company also must not permit its interest coverage ratio as of the last day of a fiscal quarter to be less than 2.50 to 1.00. The note purchase agreements generally define the interest coverage ratio as the ratio of EBITDA for the four prior fiscal quarters to interest charges for the same period. The Company was in compliance with these covenants at September 30, 2018.
Number of compliance covenants | Covenant		2
4.27% Series B Senior Notes, Due June 14th 2022 | Minimum
Debt Instrument [Line Items]
Debt instrument, prepayment percentage of aggregate principal amount		5.00%
Interest coverage ratio that the Company must exceed on the last day of the fiscal quarter		2.50%
4.27% Series B Senior Notes, Due June 14th 2022 | Maximum
Debt Instrument [Line Items]
Debt to capitalization ratio, percentage the Company must not exceed at any time		50.00%
3.95% Senior Notes Due 2028
Debt Instrument [Line Items]
Debt maturity date	Feb. 01, 2028
Long-term debt, aggregate principal amount	$ 525,000,000
Debt interest rate	3.95%			3.95%		3.95%
Debt instrument, redemption percentage	100.00%
Proceeds from borrowings, before offering expenses	$ 521,000,000
Debt payment term						The Company pays interest on the 2028 Notes on February 1 and August 1 of each year.
Debt instrument redemption description						The Company, at its option, may redeem the Notes in whole or in part, at any time or from time to time at a redemption price equal to 100% of the principal amount of such 2028 Notes to be redeemed, plus accrued and unpaid interest, if any, on those 2028 Notes to the redemption date, plus a make-whole premium. Additionally, commencing on November 1, 2027, the Company, at its option, may redeem the 2028 Notes in whole or in part, at a redemption price equal to 100% of the principal amount of the 2028 Notes to be redeemed, plus accrued and unpaid interest, if any, on those 2028 Notes to the redemption date.
Debt instrument redemption upon the occurrence of change of control, description						Upon the occurrence of a change of control, as defined in the indenture, each holder of the 2028 Notes may require the Company to purchase all or a portion of such holder’s 2028 Notes at a price equal to 101% of their principal amount, plus accrued and unpaid interest, if any, to, but excluding, the repurchase date.
Redemption price percentage of principal amount of debt instrument on change of control	101.00%
Previous Credit Agreement
Debt Instrument [Line Items]
Interest expense related to amortization of debt issuance costs						$ 317,000