Long Term Debt - Senior Notes - Additional Information (Detail)			3 Months Ended		6 Months Ended
	Jun. 14, 2012 USD ($) Covenant	Oct. 05, 2010 USD ($) Covenant	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 12, 2016 USD ($)
Debt Instrument [Line Items]
Debt maturity date					Sep. 27, 2017
Interest expense related to amortization of debt issuance costs			$ 710,000	$ 746,000	$ 1,300,000	$ 1,500,000
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.
Prepayment terms, percent of principal before accrued and unpaid interest and "make-whole" premium		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 June 30, 2017.
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.
Prepayment terms, percent of principal before accrued and unpaid interest and "make-whole" premium	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 June 30, 2017.
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%
Senior Unsecured Facility | Bridge Facility
Debt Instrument [Line Items]
Maximum aggregate principal amount							$ 150,000,000