Fair Value of Financial Instruments - Company Estimates Receivable from Affiliate using Market Quotations for U.S. Treasury Securities (Details) - Affiliated entity - Significant other observable inputs (Level 2) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest rate (percent)	7.00%	7.00%
Carrying value
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable from affiliate	$ 70.0	$ 70.0
Fair value
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable from affiliate	$ 71.1	$ 72.6