Debt Obligations - Additional Information on Repurchase of Outstanding Unsecured Notes (Details) - USD ($)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Extinguishment Of Debt [Line Items]
Principal		$ 325,000,000	$ 0	$ 0
Repurchase Amount		628,590,000	149,919,000	0
Loss on early extinguishment of debt		(3,933,000)	$ (66,590,000)	$ 0
$250.0M 5.400% Guaranteed Notes due 2014 [Member]
Extinguishment Of Debt [Line Items]
Principal		325,000,000
Repurchase Amount	[1]	330,792,000
Loss on early extinguishment of debt	[2]	$ (3,933,000)

[1] Includes prepayment penalties with respect to the redemption of debt.

[2] Includes unamortized balance of the original issue discount and deferred financing costs.