Fair Value of Financial Instruments - Embedded Derivatives Fair Value Inputs (Detail) - Embedded Credit Derivative - GSE CRT - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information
Net embedded derivative	$ 42,932	$ 45,400	$ 31,243	$ 17,095
Market Comparables, Vendor Pricing and Internal Models | Level 3 | Minimum
Fair Value Inputs, Assets, Quantitative Information
Weighted average life	2 years 6 months	2 years 7 months
Market Comparables, Vendor Pricing and Internal Models | Level 3 | Maximum
Fair Value Inputs, Assets, Quantitative Information
Weighted average life	6 years 7 months	6 years 9 months
Market Comparables, Vendor Pricing and Internal Models | Level 3 | Weighted Average
Fair Value Inputs, Assets, Quantitative Information
Weighted average life	4 years 7 months	4 years 9 months