Segment Information - Summary of Segment Information (Parenthetical) (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting [Abstract]
Taxable-equivalent adjustment	$ 5,733,000	$ 8,828,000	$ 15,939,000	$ 25,563,000