Debt - Summary of Debt (Parenthetical) (Detail) $ in Thousands, £ in Millions		Dec. 31, 2017 USD ($)	Dec. 31, 2017 GBP (£)	Sep. 07, 2017	Mar. 04, 2017	Dec. 31, 2016 USD ($)	Apr. 17, 2014	Oct. 10, 2013	Feb. 17, 2012
Debt Instrument [Line Items]
Debt		$ 4,941,310
6.375% Senior Unsecured Notes Due 2022 [Member]
Debt Instrument [Line Items]
Senior unsecured notes, interest rate		6.375%	6.375%			6.375%			6.375%
Debt						$ 350,000
5.750% Senior Unsecured Notes Due 2020 [Member]
Debt Instrument [Line Items]
Senior unsecured notes, interest rate		5.75%	5.75%		5.75%	5.75%		5.75%
Debt	[1]					$ 210,340
4.000% Senior Unsecured Notes due 2022 [Member]
Debt Instrument [Line Items]
Senior unsecured notes, interest rate		4.00%	4.00%			4.00%
Debt	[1]	$ 600,250				$ 525,850
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%
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%
Debt	[1]	$ 600,250
5.250% Senior Unsecured Notes Due 2026 [Member]
Debt Instrument [Line Items]
Senior unsecured notes, interest rate		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
GBP-denominated Borrowings [Member]
Debt Instrument [Line Items]
Debt | £			£ 8

[1] These notes are Euro-denominated and reflect the exchange rate at December 31, 2017 and December 31, 2016, respectively.