Debt and Credit Facilities (Borrowing Obligations) (Detail) - USD ($) $ in Thousands	1 Months Ended			3 Months Ended	6 Months Ended
	Sep. 30, 2018	Nov. 30, 2017	Jan. 31, 2017	Mar. 31, 2018	Mar. 31, 2019	Dec. 22, 2016	Jun. 21, 2016	Oct. 22, 2012	Aug. 14, 2012
Debt Instrument [Line Items]
Document Period End Date					Mar. 31, 2019
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months					$ 0
Long-term Debt, Maturities, Repayments of Principal in Year Two					0
Long-term Debt, Maturities, Repayments of Principal in Year Three					0
Long-term Debt, Maturities, Repayments of Principal in Year Four					310,463
Long-term Debt, Maturities, Repayments of Principal in Year Five					676,488
Long-term Debt, Maturities, Repayments of Principal in Rolling after Year Five					1,150,000
Total debt	$ 2,185,361				1,911,185
Less: current portion	0				0
Total long-term debt	2,185,361				1,911,185
Debt Issuance Costs, Noncurrent, Net	(843)				(677)
Long-term Debt, Gross					2,136,951
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net					(225,766)
5.625% Senior Notes due 2026 [Member]
Debt Instrument [Line Items]
Senior Notes, net of unamortized premium	494,915				495,210	$ 500,000
5.375% Senior Notes due August 15, 2020
Debt Instrument [Line Items]
Senior Notes, net of unamortized premium	298,759				0			$ 350,000	$ 700,000
Repayments of Debt	150,000		$ 600,000
6.0% Senior Notes due 2024 [Member]
Debt Instrument [Line Items]
Senior Notes, net of unamortized premium	298,220				298,375		$ 300,000
Convertible Debentures One Percent Due Twenty Thirty Five [Member]
Debt Instrument [Line Items]
Senior Notes, net of unamortized premium	553,973				567,128
Convertible Debentures Two Point Seven Five Percent Due November One Twenty Thirty One [Member]
Debt Instrument [Line Items]
Convertible Debentures, net of unamortized discount	46,568				46,568
Repayments of Debt		$ 331,200		$ 331,200
1.25% Convertible Debentures due 2025 [Member]
Debt Instrument [Line Items]
Convertible Debentures, net of unamortized discount	263,863				269,485
Convertible Debentures One Point Five Percent Due November One Twenty Thirty Five [Member] [Member]
Debt Instrument [Line Items]
Convertible Debentures, net of unamortized discount	$ 229,906				235,096
Senior Notes [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months					0
Long-term Debt, Maturities, Repayments of Principal in Year Two					0
Long-term Debt, Maturities, Repayments of Principal in Year Three					0
Long-term Debt, Maturities, Repayments of Principal in Year Four					0
Long-term Debt, Maturities, Repayments of Principal in Year Five					0
Long-term Debt, Maturities, Repayments of Principal in Rolling after Year Five					800,000
Total debt					792,908
Long-term Debt, Gross					800,000
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net					(7,092)
Convertible Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months					0
Long-term Debt, Maturities, Repayments of Principal in Year Two					0
Long-term Debt, Maturities, Repayments of Principal in Year Three					0
Long-term Debt, Maturities, Repayments of Principal in Year Four					310,463
Long-term Debt, Maturities, Repayments of Principal in Year Five					676,488
Long-term Debt, Maturities, Repayments of Principal in Rolling after Year Five					350,000
Total debt					1,118,277
Long-term Debt, Gross					1,336,951
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net					$ (218,674)