Debt - 5.500% Senior Unsecured Notes Due 2024 - Additional Information (Detail) - 5.500% Senior Unsecured Notes Due 2024 [Member] - USD ($)		12 Months Ended
	Apr. 17, 2014	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Senior unsecured notes face amount	$ 300,000,000
Senior unsecured notes, maturity date	May 01, 2024
Senior unsecured notes, interest rate	5.50%	5.50%	5.50%
Senior notes, repurchased price percentage on principal amount plus accrued and unpaid interest	101.00%
Senior unsecured notes, payable term		Interest on the notes is payable semi-annually on May 1 and November 1 of each year.
Senior notes, earliest redemption date	May 01, 2019
Senior unsecured notes, redemption description		We may redeem some or all of the notes at any time prior to May 1, 2019 at a “make-whole” redemption price. On or after May 1, 2019, we may redeem some or all of the notes at a premium that will decrease over time. In addition, at any time prior to May 1, 2017, we may redeem up to 35% of the aggregate principal amount of the notes using the proceeds of one or more equity offerings. In the event of a change of control, each holder of the notes may require us to repurchase some or all of our 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.
Maximum [Member]
Debt Instrument [Line Items]
Senior unsecured notes, redemption percentage on principal amount	35.00%