Derivatives and Fair Value Measurements - Fair Values of Swaps in Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	$ 11	$ 30
Liability derivatives not designated as hedging instruments	(8,272)	(5,810)
Net total not designated as hedging instruments	(8,261)	(5,780)
Swaps [Member] | Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments	11	11
Liability derivatives not designated as hedging instruments	(5,875)	(4,468)
Net total not designated as hedging instruments	(5,864)	(4,457)
Swaps [Member] | Other Deferred Credits [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives not designated as hedging instruments		19
Liability derivatives not designated as hedging instruments	(2,397)	(1,342)
Net total not designated as hedging instruments	$ (2,397)	$ (1,323)