Long-term debt (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Line Items]
Total principal long-term debt	$ 4,078,392	$ 4,214,067
Debt issuance costs	(27,615)	(31,800)
Other (fair market value adjustments and discounts)	(7,382)	(12,605)
Total long-term debt	4,043,395	4,169,662
Less current portion of long-term debt maturities of VIE loans	646	646
Long-term debt, net of current portion	4,042,749	4,169,016
Unsecured floating rate term loan due quarterly through August 2018
Debt Disclosure [Line Items]
Total principal long-term debt	52,100	83,700
VIE unsecured floating rate term loans due quarterly through December 2018
Debt Disclosure [Line Items]
Total principal long-term debt	1,292	1,938
Unsecured floating rate term loan due quarterly through June 2020
Debt Disclosure [Line Items]
Total principal long-term debt	140,000	180,000
Unsecured floating rate term loan due quarterly through September 2020
Debt Disclosure [Line Items]
Total principal long-term debt	285,000	0
Borrowings under revolving credit agreement expiring June 2020
Debt Disclosure [Line Items]
Total principal long-term debt	635,000	720,000
Unsecured notes bearing fixed rate interest at 10% due April 2016
Debt Disclosure [Line Items]
Total principal long-term debt	0	193,429
Unsecured notes bearing fixed rate interest at 7.125% due September 2018
Debt Disclosure [Line Items]
Total principal long-term debt	0	70,000
Unsecured notes bearing fixed rate interest at 5.125% due October 2019
Debt Disclosure [Line Items]
Total principal long-term debt	600,000	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