Repurchase Agreements - Maturities (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	$ 7,146,996	$ 13,427,545
Overnight
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	40,836	0
1 to 29 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	3,634,255	9,709,387
30 to 59 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	502,333	800,648
60 to 89 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	633,939	608,520
90 to 119 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	113,370	0
Greater than or equal to 120 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	$ 2,222,263	$ 2,308,990