Repurchase Agreements - Maturities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	$ 9,127,048	$ 7,250,452
Overnight
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	0	0
1 to 29 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	4,757,404	4,745,342
30 to 59 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	1,915,294	1,206,769
60 to 89 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	1,460,284	592,865
90 to 119 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	90,183	0
Greater than or equal to 120 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	$ 903,883	$ 705,476