Segment Information, Nature of Operations, and Certain Concentrations - Interest Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting, Other Significant Reconciling Item [Line Items]
Consolidated interest income	$ 12,404	$ 8,680	$ 7,696
Mortgage Banking:
Segment Reporting, Other Significant Reconciling Item [Line Items]
Interest income	7,850	7,569	6,485
Profit before taxes:
Segment Reporting, Other Significant Reconciling Item [Line Items]
Interest income	7,850	7,569	6,485
Other unallocated interest income
Segment Reporting, Other Significant Reconciling Item [Line Items]
Other unallocated interest income	$ 4,554	$ 1,111	$ 1,211