Segment Information - Summary of Segment Information (Parenthetical) (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting [Abstract]
Taxable-equivalent adjustment	$ 5,397,000	$ 8,736,000	$ 10,206,000	$ 16,735,000