Fair Value Measurements - Summary of Information about Described Model Inputs Used to Determine Fair Value of Each Class of Credit Derivatives (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2017
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Measurement Process Description For Credit Derivatives Hypothetical Incremental Fee	0.05%
Fair Value Measurement Process Description For Credit Derivatives Hypothetical Incremental Fee With Credit Deterioration	0.15%
Minimum [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Measurement Process Description For Credit Derivatives Base Fee	0.20%
Fair Value Measurement Process Description For Credit Derivatives Base Reference Obligation Spread	0.80%
Maximum [Member]
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Measurement Process Description For Credit Derivatives Revised Hypothetical Fee	0.25%
Fair Value Measurement Process Description For Credit Derivatives Revised Hypothetical Reference Obligation Spread Two	1.00%