Schedule of Repurchase Agreements Obligation by Type of Collateral Pledged (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase Agreements	$ 142,519	$ 118,840
Gross Amount of Recognized Liabilities for Repurchase Agreements in Note 11. "Offsetting of Assets and Liabilities"	142,519	118,840
Amounts Related to Agreements Not Included in Offsetting Disclosure in Note 11. "Offsetting of Assets and Liabilities"	0	0
Asset-backed Securities
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase Agreements	142,519	118,840
Overnight and Continuous
Assets Sold under Agreements to Repurchase [Line Items]
Amounts Related to Agreements Not Included in Offsetting Disclosure in Note 11. "Offsetting of Assets and Liabilities"	0	0
Up to 30 Days
Assets Sold under Agreements to Repurchase [Line Items]
Amounts Related to Agreements Not Included in Offsetting Disclosure in Note 11. "Offsetting of Assets and Liabilities"	0	0
Up to 30 Days | Asset-backed Securities
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase Agreements	25,185	22,756
30 - 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Amounts Related to Agreements Not Included in Offsetting Disclosure in Note 11. "Offsetting of Assets and Liabilities"	0	0
30 - 90 Days | Asset-backed Securities
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase Agreements	69,960	96,084
Greater than 90 Days
Assets Sold under Agreements to Repurchase [Line Items]
Amounts Related to Agreements Not Included in Offsetting Disclosure in Note 11. "Offsetting of Assets and Liabilities"	0	$ 0
Greater than 90 Days | Asset-backed Securities
Assets Sold under Agreements to Repurchase [Line Items]
Repurchase Agreements	$ 47,374