Secured Financing Agreements - Maturities (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	$ 5,944,201	$ 13,427,545
Repurchase Agreements
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	5,743,417	13,427,545
Repurchase Agreements | Overnight
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	0	0
Repurchase Agreements | 1 to 29 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	2,311,122	9,709,387
Repurchase Agreements | 30 to 59 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	296,041	800,648
Repurchase Agreements | 60 to 89 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	65,390	608,520
Repurchase Agreements | 90 to 119 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	0	0
Repurchase Agreements | 120 to 180 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	191,904	809,077
Repurchase Agreements | 180 days to 1 year
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	1,497,192	580,886
Repurchase Agreements | 1 to 2 years
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	187,187	427,981
Repurchase Agreements | 2 to 3 years
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	400,000	0
Repurchase Agreements | Greater than 3 years
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	$ 794,581	$ 491,046
Repurchase Agreements | Weighted Average Borrowing Rates
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	2.93%	2.52%
Repurchase Agreements | Weighted Average Borrowing Rates | 1 to 29 days
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	0.48%	2.26%
Repurchase Agreements | Weighted Average Borrowing Rates | 30 to 59 days
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	2.55%	2.96%
Repurchase Agreements | Weighted Average Borrowing Rates | 60 to 89 days
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	2.37%	3.00%
Repurchase Agreements | Weighted Average Borrowing Rates | 120 to 180 days
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	1.96%	3.38%
Repurchase Agreements | Weighted Average Borrowing Rates | 180 days to 1 year
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	5.34%	3.42%
Repurchase Agreements | Weighted Average Borrowing Rates | 1 to 2 years
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	2.92%	3.28%
Repurchase Agreements | Weighted Average Borrowing Rates | 2 to 3 years
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	7.00%
Repurchase Agreements | Weighted Average Borrowing Rates | Greater than 3 years
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	3.91%	3.20%
Repurchase Agreements | Minimum | 1 to 29 days
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	0.21%	1.90%
Repurchase Agreements | Minimum | 30 to 59 days
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	1.97%	2.15%
Repurchase Agreements | Minimum | 60 to 89 days
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	2.15%	2.59%
Repurchase Agreements | Minimum | 120 to 180 days
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	1.66%	3.06%
Repurchase Agreements | Minimum | 180 days to 1 year
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	1.68%	3.26%
Repurchase Agreements | Minimum | 1 to 2 years
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	1.68%	3.19%
Repurchase Agreements | Minimum | 2 to 3 years
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	7.00%
Repurchase Agreements | Minimum | Greater than 3 years
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	1.58%	3.19%
Repurchase Agreements | Maximum | 1 to 29 days
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	2.74%	3.62%
Repurchase Agreements | Maximum | 30 to 59 days
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	2.67%	3.52%
Repurchase Agreements | Maximum | 60 to 89 days
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	2.43%	3.35%
Repurchase Agreements | Maximum | 120 to 180 days
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	2.93%	3.46%
Repurchase Agreements | Maximum | 180 days to 1 year
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	8.74%	3.51%
Repurchase Agreements | Maximum | 1 to 2 years
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	4.50%	3.30%
Repurchase Agreements | Maximum | 2 to 3 years
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	7.00%
Repurchase Agreements | Maximum | Greater than 3 years
Assets Sold under Agreements to Repurchase [Line Items]
Borrowing rates (as a percent)	5.56%	3.20%