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Goodwill and Intangible Assets - Narrative (Detail) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]      
Goodwill $ 147,019,000   $ 147,019,000
Changes in carrying amount of goodwill 0   $ 0
Amortization of intangibles $ 1,413,000 $ 1,725,000