Schedule of Debt Outstanding (Parenthetical) (Detail) - USD ($)	1 Months Ended		12 Months Ended
	Sep. 30, 2015	Apr. 30, 2013	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Unamortized debt issuance costs			$ 11,813,000	$ 9,831,000
Term Loan Due in July 2020
Debt Instrument [Line Items]
Interest payable dates			interest payable periodically at variable interest rates.
Debt interest rate			3.10%	2.30%
Debt maturity date			Jul. 31, 2020
Unamortized debt issuance costs			$ 300,000	$ 400,000
Senior Notes 4.625% Due 2023
Debt Instrument [Line Items]
Interest payable dates			interest payable semi-annually in May and November at 4.625%.
Debt maturity date		May 01, 2023	May 01, 2023
Unamortized debt issuance costs			$ 3,300,000	3,900,000
Stated interest rate		4.625%	4.625%
Senior Notes 4.125% Due 2020
Debt Instrument [Line Items]
Interest payable dates			interest payable semi-annually in April and October at 4.125%.
Debt maturity date		Apr. 01, 2020	Apr. 01, 2020
Unamortized debt issuance costs			$ 1,900,000	2,700,000
Stated interest rate		4.125%	4.125%
Senior Notes 5.0% Due 2022
Debt Instrument [Line Items]
Interest payable dates			interest payable semi-annually in April and October at 5.0%.
Debt maturity date	Oct. 15, 2022		Oct. 15, 2022
Unamortized debt issuance costs			$ 2,300,000	2,800,000
Stated interest rate	5.00%		5.00%
Senior Notes 4.75% Due 2027
Debt Instrument [Line Items]
Interest payable dates			interest payable semi-annually in April and October at 4.75%.
Debt maturity date			Oct. 15, 2027
Unamortized debt issuance costs			$ 4,000,000
Stated interest rate			4.75%
Revolving Credit Facility | $900.0 Million Revolving Credit Facility
Debt Instrument [Line Items]
Line of credit facility maximum borrowing capacity			$ 900,000,000	$ 900,000,000
Revolving Credit Facility maturity date			Jul. 31, 2020
Weighted average rate			3.10%	2.20%
Interest payable dates			interest payable periodically at variable interest rates.