Investments - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2018
Minimum [Member] | Interest-bearing Deposits [Member]
Schedule of Investments [Line Items]
Deposits maturity period	3 months
Minimum [Member] | Debt [Member] | U.S. Treasury Securities [Member]
Schedule of Investments [Line Items]
US Treasury securities maturity period	3 months
Maximum [Member] | Interest-bearing Deposits [Member]
Schedule of Investments [Line Items]
Deposits maturity period	1 year
Maximum [Member] | Debt [Member] | U.S. Treasury Securities [Member]
Schedule of Investments [Line Items]
US Treasury securities maturity period	1 year