Long-term debt (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Disclosure [Line Items]
Total principal long-term debt	$ 2,960,000	$ 3,031,146
Debt issuance costs	(15,458)	(20,551)
Other (fair market value adjustments and discounts)	(76)	(2,902)
Total long-term debt	2,944,466	3,007,693
Less current portion of long-term debt maturities of VIE loans	0	646
Long-term debt, net of current portion	2,944,466	3,007,047
Unsecured floating rate term loan paid quarterly through August 2018
Debt Disclosure [Line Items]
Total principal long-term debt	0	20,500
VIE unsecured floating rate term loans paid quarterly through December 2018
Debt Disclosure [Line Items]
Total principal long-term debt	0	646
Unsecured floating rate term loan due quarterly through June 2020
Debt Disclosure [Line Items]
Total principal long-term debt	60,000	100,000
Unsecured floating rate term loan due quarterly through September 2020
Debt Disclosure [Line Items]
Total principal long-term debt	165,000	225,000
Borrowings under revolving credit agreement expiring June 2023
Debt Disclosure [Line Items]
Total principal long-term debt	50,000	0
Unsecured notes bearing fixed rate interest at 5.125% due October 2019
Debt Disclosure [Line Items]
Total principal long-term debt	320,000	320,000
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