Variable Interest Entities - Summary of Quantitative Information Related to Significant Involvement in Unconsolidated Variable Interest Entities (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Variable Interest Entity [Line Items]
Aggregate Assets	$ 26,234,973	$ 19,662,324
Aggregate Liabilities	22,037,118	16,298,491
Risk Of Loss	26,234,973		$ 19,882,539
Trust Preferred Securities [Member]
Variable Interest Entity [Line Items]
Risk Of Loss	10,606	9,261
Trust Preferred Securities [Member] | Variable Interest Entity, Not Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Aggregate Assets	295,263	257,941
Aggregate Liabilities	$ 284,657	$ 248,680