Financing Arrangements - Summary of Information of Outstanding Financing Arrangement (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Assets Sold under Agreements to Repurchase [Line Items]
Amount Outstanding	$ 159,126
Amount Available	$ 45,874
WF-1 Facility [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Weighted Average Rate	4.49%	3.75%
Amount Outstanding	$ 31,305	$ 23,250
Amount Available	$ 43,695	$ 51,750
Weighted Average Term	3 years 4 months 24 days	3 years 1 month 6 days
GS-1 Facility [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Weighted Average Rate	4.35%
Amount Outstanding	$ 127,821
Amount Available	$ 2,179
Weighted Average Term	3 years 9 months 18 days