Fair Value of Financial Instruments - Company Estimates Receivable from Affiliate using Market Quotations for U.S. Treasury Securities (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Document Period End Date		Sep. 30, 2020
Affiliated Entity [Member] | Level 2 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable interest rate (percent)			7.00%
Note Receivable from Affiliate, 2019 [Member] | Affiliated Entity [Member] | Level 2 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable interest rate (percent)	4.05%	4.05%
Reported Value Measurement [Member] | Note Receivable from Affiliate, 2019 [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] | Note Receivable from Affiliate, 2019 [Member] | Affiliated Entity [Member] | Level 2 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable from affiliate		$ 82.4		$ 74.6