Repurchase Agreements - Maturities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	$ 7,250,452	$ 5,600,903
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	4,745,342	2,947,604
30 to 59 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	1,206,769	958,956
60 to 89 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	592,865	407,625
90 to 119 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	0	559,533
Greater than or equal to 120 days
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	$ 705,476	$ 727,185