Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Transfers between Level 1 and 2	$ 0	$ 0
Own credit risk	0.30%	0.50%
Long-term forward rate assumption, minimum	25.00%
Long-term forward rate assumption, maximum	30.00%
Market based implied volatility inputs	5 years
Increase in interest rate		0.11%
Increase in own credit risk		0.10%