Loans (Tier Risk Rating System) - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2018
Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Repayment period of loan with a reasonable reduction of principal balance	2 years
Good [Member] | Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Leverage position	1.50%
3 [Member] | Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Leverage position	2.00%