Repurchase Agreements - Maturities (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	$ 7,439,339	$ 8,455,381
1 to 29 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	3,312,902	2,652,717
30 to 59 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	2,501,513	1,371,856
60 to 89 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	246,970	656,915
90 to 119 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	430,026	2,068,740
Greater than or equal to 120 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase agreements outstanding	$ 947,928	$ 1,705,153