Fair Value of Financial Instruments - Company Estimates Receivable from Affiliate using Market Quotations for U.S. Treasury Securities (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Document Period End Date	Sep. 30, 2018
Affiliated Entity [Member] | Level 2 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable interest rate (percent)	7.00%	7.00%
Reported Value Measurement [Member] | Affiliated Entity [Member] | Level 2 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable from affiliate	$ 70.0	$ 70.0
Estimate of Fair Value Measurement [Member] | Affiliated Entity [Member] | Level 2 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable from affiliate	$ 72.5	$ 72.6