Variable Interest Entities - Summary of Variable Interest Entities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Variable Interest Entity [Line Items]
Investment securities available-for-sale	$ 3,600,731	$ 1,368,384
Accrued expenses and other liabilities	282,905	131,367
Other assets	282,276	154,992
Total assets	19,844,240	11,776,012
Liabilities	17,156,155	9,763,418
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Maximum loss	241,418	32,466
Total assets	209,752	53,880
Liabilities	19,775	21,414
Variable Interest Entity, Not Primary Beneficiary | Multifamily Loan Securitization
Variable Interest Entity [Line Items]
Investment securities available-for-sale	114,460	0
Accrued expenses and other liabilities	540	0
Variable Interest Entity, Not Primary Beneficiary | Multifamily Loan Securitization, Asset
Variable Interest Entity [Line Items]
Maximum loss	114,460	0
Variable Interest Entity, Not Primary Beneficiary | Multifamily Loan Securitization, Liability
Variable Interest Entity [Line Items]
Maximum loss	50,901	0
Variable Interest Entity, Not Primary Beneficiary | Affordable Housing Partnership
Variable Interest Entity [Line Items]
Accrued expenses and other liabilities	19,235	21,414
Other assets	95,292	53,880
Variable Interest Entity, Not Primary Beneficiary | Affordable Housing Partnership, Asset
Variable Interest Entity [Line Items]
Maximum loss	76,057	32,466
Variable Interest Entity, Not Primary Beneficiary | Affordable Housing Partnership, Liability
Variable Interest Entity [Line Items]
Maximum loss	$ 0	$ 0