Variable Interest Entities - Summary of Quantitative Information Related to Significant Involvement in Unconsolidated Variable Interest Entities (Detail) - Trust Preferred Securities [Member] - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Aggregate Assets	$ 257,518	$ 266,669
Aggregate Liabilities	248,629	257,674
Risk Of Loss	$ 8,889	$ 8,995