Debt Commitments - Debt Obligations and Letters of Credit Outstanding (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Total	$ 390.0	$ 405.0
Less: Current portion of debt	(60.0)	(40.0)
Long-term debt	$ 330.0	365.0
Credit Facility \| Unsecured Revolving Credit Facility
Debt Instrument [Line Items]
Average Interest Rate, weighted per annum	1.05%
Total	$ 25.0	0.0
Credit Facility \| Letter of Credit Subfacility
Debt Instrument [Line Items]
Outstanding letters of credit under unsecured revolving credit facility - contingent obligation	36.3	36.3
Senior Unsecured Promissory Notes
Debt Instrument [Line Items]
Total	$ 365.0
Senior Unsecured Promissory Notes \| Senior unsecured promissory notes payable, Series A
Debt Instrument [Line Items]
Average Interest Rate, fixed per annum	2.00%
Total	$ 0.0	40.0
Senior Unsecured Promissory Notes \| Senior unsecured promissory notes payable, Series B
Debt Instrument [Line Items]
Average Interest Rate, fixed per annum	2.45%
Total	$ 35.0	35.0
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	$ 70.0	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

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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