Debt - Summary of Debt (Parenthetical) (Detail) $ in Thousands, £ in Millions		Mar. 31, 2019 USD ($)	Mar. 31, 2019 GBP (£)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 GBP (£)
Debt Instrument [Line Items]
Debt		$ 4,059,602
4.000% Senior Unsecured Notes due 2022 [Member]
Debt Instrument [Line Items]
Senior unsecured notes, interest rate		4.00%	4.00%	4.00%	4.00%
Debt	[1]	$ 560,900		$ 573,350
5.500% Senior Unsecured Notes Due 2024 [Member]
Debt Instrument [Line Items]
Senior unsecured notes, interest rate		5.50%	5.50%	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%	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%	3.325%	3.325%	3.325%
Debt	[1]	$ 560,900		$ 573,350
5.250% Senior Unsecured Notes Due 2026 [Member]
Debt Instrument [Line Items]
Senior unsecured notes, interest rate		5.25%	5.25%	5.25%	5.25%
Debt		$ 500,000		$ 500,000
5.000% Senior Unsecured Notes Due 2027 [Member]
Debt Instrument [Line Items]
Senior unsecured notes, interest rate		5.00%	5.00%	5.00%	5.00%
Debt		$ 1,400,000		$ 1,400,000
GBP-denominated Borrowings [Member]
Debt Instrument [Line Items]
Debt | £			£ 29		£ 22

[1] These notes are Euro-denominated and reflect the exchange rate at March 31, 2019 and December 31, 2018, respectively.