Long-term debt - Interest Rates (Detail)	Dec. 31, 2017	Oct. 16, 2017
Unsecured notes bearing fixed rate interest at 5.125% due October 2019
Debt Disclosure [Line Items]
Interest rate stated percentage	5.125%	5.125%
Unsecured notes bearing fixed rate interest at 5.125% due July 2020
Debt Disclosure [Line Items]
Interest rate stated percentage	5.125%
Unsecured notes bearing fixed rate interest at 4.875% due September 2021
Debt Disclosure [Line Items]
Interest rate stated percentage	4.875%
Unsecured notes bearing fixed rate interest at 6.375% due October 2023
Debt Disclosure [Line Items]
Interest rate stated percentage	6.375%
Unsecured notes bearing fixed rate interest at 5.50% due September 2024
Debt Disclosure [Line Items]
Interest rate stated percentage	5.50%
Unsecured notes bearing fixed rate interest at 7.75% due June 2027
Debt Disclosure [Line Items]
Interest rate stated percentage	7.75%
Unsecured notes bearing fixed rate interest at 7.25% due September 2027
Debt Disclosure [Line Items]
Interest rate stated percentage	7.25%