Debt and Credit Facilities (Borrowing Obligations) (Detail) - USD ($) $ in Thousands	1 Months Ended			3 Months Ended
	Sep. 30, 2018	Nov. 30, 2017	Jan. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 22, 2016	Jun. 21, 2016	Oct. 22, 2012	Aug. 14, 2012
Debt Instrument [Line Items]
Document Period End Date				Dec. 31, 2018
Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months				$ 0
Long-term Debt, Maturities, Repayments of Principal in Year Two				300,000
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			2,197,734
Less: current portion	0			0
Total long-term debt	2,185,361			2,197,734
Debt Issuance Costs, Noncurrent, Net	(843)			(760)
Long-term Debt, Gross				2,436,951
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net				(239,217)
5.625% Senior Notes due 2026 [Member]
Debt Instrument [Line Items]
Senior Notes, net of unamortized premium	494,915			495,056		$ 500,000
5.375% Senior Notes due August 15, 2020
Debt Instrument [Line Items]
Senior Notes, net of unamortized premium	298,759			298,925				$ 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			298,297			$ 300,000
Convertible Debentures One Percent Due Twenty Thirty Five [Member]
Debt Instrument [Line Items]
Senior Notes, net of unamortized premium	553,973			560,507
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			266,655
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			232,486
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				300,000
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				1,091,518
Long-term Debt, Gross				1,100,000
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net				(8,482)
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				1,106,216
Long-term Debt, Gross				1,336,951
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net				$ (230,735)