Debt Commitments - Debt Obligations and Letters of Credit Outstanding (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Total	$ 350.0	$ 555.0
Less: Current portion of debt	(150.0)	(201.8)
Long-term debt	$ 200.0	353.2
Credit Facility | Unsecured Revolving Credit Facility
Debt Instrument [Line Items]
Average Interest Rate, weighted per annum	6.10%
Total	$ 90.0	225.0
Credit Facility | Letter of Credit Subfacility
Debt Instrument [Line Items]
Outstanding letters of credit under unsecured revolving credit facility - contingent obligation	$ 32.7	36.3
Senior Unsecured Promissory Notes | Senior unsecured promissory notes payable, Series C
Debt Instrument [Line Items]
Average Interest Rate, fixed per annum	3.22%
Total	$ 60.0	60.0
Senior Unsecured Promissory Notes | Senior unsecured promissory notes payable, Series D
Debt Instrument [Line Items]
Average Interest Rate, fixed per annum	2.66%
Total	$ 75.0	75.0
Senior Unsecured Promissory Notes | Senior unsecured promissory notes payable, Series E
Debt Instrument [Line Items]
Average Interest Rate, fixed per annum	2.72%
Total	$ 50.0	50.0
Senior Unsecured Promissory Notes | Senior unsecured promissory notes payable, Series F
Debt Instrument [Line Items]
Average Interest Rate, fixed per annum	1.69%
Total	$ 0.0	70.0
Senior Unsecured Promissory Notes | Senior unsecured promissory notes payable, Series G
Debt Instrument [Line Items]
Average Interest Rate, fixed per annum	2.13%
Total	$ 25.0	25.0
Senior Unsecured Promissory Notes | Senior unsecured promissory notes payable, Series H
Debt Instrument [Line Items]
Average Interest Rate, fixed per annum	2.50%
Total	$ 50.0	$ 50.0