Goodwill and Intangible Assets - Narrative (Detail) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	$ 147,019,000		$ 147,019,000		$ 147,019,000
Changes in carrying amount of goodwill			0		$ 0
Amortization of intangibles	$ 1,400,000	$ 1,700,000	$ 2,834,000	$ 3,450,000