Notes Payable - Schedule of Long Term Debt (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	May 16, 2019	Aug. 13, 2018	Aug. 28, 2012
Debt Instrument [Line Items]
Notes payable	$ 839.0	$ 838.0
Unsecured notes | 4.50% Notes, net of discount
Debt Instrument [Line Items]
Stated interest rate	4.50%		4.50%	4.50%
Notes payable	$ 443.0	443.0
Unsecured notes | 5.50% Notes, net of discount
Debt Instrument [Line Items]
Stated interest rate	5.50%				5.50%
Notes payable	$ 398.0	398.0
Line of Credit | Revolving Credit Facility | Revolving Credit Facility
Debt Instrument [Line Items]
Notes payable	2.0	$ 3.0
Unamortized debt issuance costs	$ 2.0