Additional Financial Information - Additional Information (Detail) - USD ($) $ in Thousands						3 Months Ended	6 Months Ended
Dec. 20, 2018	Aug. 07, 2018	May 07, 2018	Mar. 31, 2018	Jan. 08, 2018	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Supplemental Cash Flow Information [Abstract]
Accrued interest paid						$ 28,000	$ 34,000	$ 70,000	$ 86,000
Income taxes paid net of refunds						$ 11,000	$ 9,000	$ 18,000	$ 17,000
Special cash dividends	$ 31,250	$ 500,000	$ 300,000	$ 31,250	$ 125,000
Special cash dividends payable, date to be paid	Jan. 04, 2019	Aug. 10, 2018	May 11, 2018	Apr. 05, 2019	Jan. 12, 2018
Special cash dividends payable, date of record		Aug. 07, 2018	May 07, 2018	Mar. 31, 2019	Jan. 11, 2018

Date the declared dividend will be paid, in CCYY-MM-DD format.

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Amount of paid and unpaid cash dividends declared for classes of stock, for example, but not limited to, common and preferred.

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Date the holder must own the stock to be entitled to the dividend, in CCYY-MM-DD format.

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The amount of cash paid during the current period to foreign, federal, state, and local authorities as taxes on income, net of any cash received during the current period as refunds for the overpayment of taxes.

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Amount of cash paid for interest, excluding capitalized interest, classified as operating activity. Includes, but is not limited to, payment to settle zero-coupon bond for accreted interest of debt discount and debt instrument with insignificant coupon interest rate in relation to effective interest rate of borrowing attributable to accreted interest of debt discount.

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