Debt - Summary of Debt (Parenthetical) (Detail) $ in Thousands, £ in Millions		Sep. 30, 2018 USD ($)		Sep. 30, 2018 GBP (£)		Dec. 31, 2017 USD ($)	Dec. 31, 2017 GBP (£)	Sep. 07, 2017	Mar. 24, 2017
Debt Instrument [Line Items]
Debt		$ 4,082,553				$ 4,941,310
GBP-denominated Borrowings [Member]
Debt Instrument [Line Items]
Debt | £				£ 17			£ 8
4.000% Senior Unsecured Notes due 2022 [Member]
Debt Instrument [Line Items]
Senior unsecured notes, interest rate	[1]	4.00%		4.00%
Debt	[1]	$ 580,200				600,250
5.500% Senior Unsecured Notes Due 2024 [Member]
Debt Instrument [Line Items]
Senior unsecured notes, interest rate		5.50%		5.50%
Debt		$ 300,000				300,000
6.375% Senior Unsecured Notes due 2024 [Member]
Debt Instrument [Line Items]
Senior unsecured notes, interest rate		6.375%		6.375%
Debt		$ 500,000				500,000
3.325% Senior Unsecured Notes Due 2025 [Member]
Debt Instrument [Line Items]
Senior unsecured notes, interest rate		3.325%	[1]	3.325%	[1]				3.325%
Debt	[1]	$ 580,200				600,250
5.000% Senior Unsecured Notes Due 2027 [Member]
Debt Instrument [Line Items]
Senior unsecured notes, interest rate		5.00%		5.00%				5.00%
Debt		$ 1,400,000				1,400,000
5.250% Senior Unsecured Notes Due 2026 [Member]
Debt Instrument [Line Items]
Senior unsecured notes, interest rate		5.25%		5.25%
Debt		$ 500,000				$ 500,000

[1] These notes are Euro-denominated and reflect the exchange rate at September 30, 2018 and December 31, 2017, respectively.