Income Taxes - Additional Information (Detail) - USD ($)		3 Months Ended		6 Months Ended		12 Months Ended
	Dec. 22, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income tax expense		$ 7,217,000	$ 8,500,000	$ 13,462,000	$ 16,105,000
Effective tax rates on pre-tax income		16.09%	23.13%	15.72%	22.70%
Federal statutory tax rate				21.00%		35.00%
Restatement of deferred tax liability due to change in tax rate						$ 7,650,000
Change in deferred tax assets and liabilities related to reclassification from accumulated other comprehensive income to retained earnings	$ 5,759,000