Debt - 5.250% Senior Unsecured Notes Due 2026 - Additional Information (Detail) - 5.250% Senior Unsecured Notes Due 2026 [Member] - USD ($)		12 Months Ended
	Jul. 22, 2016	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Senior unsecured notes, interest rate		5.25%	5.25%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Senior unsecured notes face amount	$ 500,000,000
Senior unsecured notes, payable term		Interest on the notes is payable on February 1 and August 1 of each year.
Senior unsecured notes, interest rate	5.25%
Senior unsecured notes, maturity date	Aug. 01, 2026
Senior notes, earliest redemption date	Aug. 01, 2021
Senior unsecured notes, redemption description		We may redeem some or all of the notes at any time prior to August 1, 2021 at a “make whole” redemption price. On or after August 1, 2021, we may redeem some or all of the notes at a premium that will decrease over time. In addition, at any time prior to August 1, 2019, we may redeem up to 35% of the notes at a redemption price equal to 105.25% of the aggregate principal amount thereof, plus accrued and unpaid interest thereon, using proceeds from one or more equity offerings. In the event of a change in control, each holder of the notes may require us to repurchase some or all of the notes at a repurchase price equal to 101% of the aggregate principal amount of the notes plus accrued and unpaid interest to the date of purchase.
Debt instrument, redemption price percentage	105.25%
Senior notes, repurchased price percentage on principal amount plus accrued and unpaid interest	101.00%
Revolving Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Senior unsecured notes, redemption percentage on principal amount	35.00%