Long-term debt (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Line Items]
Total principal long-term debt	$ 3,031,146	$ 4,078,392
Debt issuance costs	(20,551)	(27,615)
Other (fair market value adjustments and discounts)	(2,902)	(7,382)
Total long-term debt	3,007,693	4,043,395
Less current portion of long-term debt maturities of VIE loans	646	646
Long-term debt, net of current portion	3,007,047	4,042,749
Unsecured floating rate term loan due quarterly through August 2018
Debt Disclosure [Line Items]
Total principal long-term debt	20,500	52,100
VIE unsecured floating rate term loans due quarterly through December 2018
Debt Disclosure [Line Items]
Total principal long-term debt	646	1,292
Unsecured floating rate term loan due quarterly through June 2020
Debt Disclosure [Line Items]
Total principal long-term debt	100,000	140,000
Unsecured floating rate term loan due quarterly through September 2020
Debt Disclosure [Line Items]
Total principal long-term debt	225,000	285,000
Borrowings under revolving credit agreement expiring June 2020
Debt Disclosure [Line Items]
Total principal long-term debt	0	635,000
Unsecured notes bearing fixed rate interest at 5.125% due October 2019
Debt Disclosure [Line Items]
Total principal long-term debt	320,000	600,000
Unsecured notes bearing fixed rate interest at 5.125% due July 2020
Debt Disclosure [Line Items]
Total principal long-term debt	600,000	600,000
Unsecured notes bearing fixed rate interest at 4.875% due September 2021
Debt Disclosure [Line Items]
Total principal long-term debt	350,000	350,000
Unsecured notes bearing fixed rate interest at 6.375% due October 2023
Debt Disclosure [Line Items]
Total principal long-term debt	650,000	650,000
Unsecured notes bearing fixed rate interest at 5.50% due September 2024
Debt Disclosure [Line Items]
Total principal long-term debt	325,000	325,000
Unsecured notes bearing fixed rate interest at 7.75% due June 2027
Debt Disclosure [Line Items]
Total principal long-term debt	200,000	200,000
Unsecured notes bearing fixed rate interest at 7.25% due September 2027
Debt Disclosure [Line Items]
Total principal long-term debt	$ 240,000	$ 240,000