Financial Guarantees - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Financial Guarantees [Line Items]
Term of standby letters of credit, years	1 year
Notional amount of standby letters of credit	$ 85,700,000	$ 68,700,000
Carrying value of letters of credit	400,000	400,000
Notional amount	1,034,000,000	$ 708,500,000
Interest Rate Swap [Member]
Financial Guarantees [Line Items]
Notional amount	$ 30,800,000