Debt (Details) $ in Millions	1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 11, 2018 USD ($)	Nov. 03, 2018 USD ($) CreditFacility	Nov. 03, 2018 USD ($) CreditFacility	Feb. 03, 2018 USD ($) CreditFacility
Line of Credit Facility [Line Items]
Number of unsecured credit facilities | CreditFacility		2	2	2
Available borrowings under credit facilities		$ 100.0	$ 100.0	$ 60.0
Days borrowings incurred against facilities		4 days	4 days	7 days
Average borrowings outstanding		$ 25.0	$ 25.0	$ 4.1
Maximum borrowings outstanding	$ 50.0	$ 25.0	$ 25.0	$ 4.9
Average interest rate on outstanding borrowings		3.81%	3.81%	2.78%
Debt outstanding at period end		$ 50.0	$ 50.0
Regions LOC [Member]
Line of Credit Facility [Line Items]
Expiration date of renewed facility		Oct. 29, 2021		Apr. 30, 2018
Maximum borrowing capacity under renewed facility		$ 50.0	50.0	$ 30.0
Line of Credit Facility, Interest Rate Description		one month LIBOR plus 1.5%		one month LIBOR plus 2.5%
Description of variable interest rate basis		one month LIBOR		one month LIBOR
Basis spread on variable interest rate (in hundredths)		1.50%		2.50%
Debt outstanding at period end		$ 0.0	0.0
Bank of America LOC [Member]
Line of Credit Facility [Line Items]
Expiration date of renewed facility		Oct. 29, 2021		Mar. 31, 2018
Maximum borrowing capacity under renewed facility		$ 50.0	50.0	$ 30.0
Line of Credit Facility, Interest Rate Description		an interest rate agreed upon between the lender and us at the time a loan is made		an interest rate agreed upon between the lender and us at the time a loan is made
Debt outstanding at period end		$ 25.0	$ 25.0