Debt Commitments - Debt Obligations and Letters of Credit Outstanding (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Total	$ 240.0	$ 260.0
Less: Current portion of debt	(115.0)	(60.0)
Long-term debt	$ 125.0	200.0
Credit Facility | Unsecured Revolving Credit Facility
Debt Instrument [Line Items]
Average Interest Rate, weighted per annum	5.88%
Total	$ 40.0	0.0
Credit Facility | Letter of Credit Subfacility
Debt Instrument [Line Items]
Outstanding letters of credit under unsecured revolving credit facility - contingent obligation	$ 31.2	32.7
Senior Unsecured Promissory Notes | Senior unsecured promissory notes payable, Series C
Debt Instrument [Line Items]
Average Interest Rate, fixed per annum	3.22%
Total	$ 0.0	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 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