Debt Commitments - Debt Obligations and Letters of Credit Outstanding (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total debt	$ 500.0	$ 415.0
Less: Current portion of debt	(3.0)	(3.0)
Long-term debt	497.0	412.0
Credit Facility \| Unsecured Revolving Credit Facility
Debt Instrument [Line Items]
Total debt	365.0	280.0
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 \| 2.00% Senior Unsecured Promissory Note Payable
Debt Instrument [Line Items]
Fixed interest rate per annum	2.00%
Total debt	$ 40.0	40.0
Senior Unsecured Promissory Notes \| 2.45% Senior Unsecured Promissory Note Payable
Debt Instrument [Line Items]
Fixed interest rate per annum	2.45%
Total debt	$ 35.0	35.0
Senior Unsecured Promissory Notes \| 3.22% Senior Unsecured Promissory Note Payable
Debt Instrument [Line Items]
Fixed interest rate per annum	3.22%
Total debt	$ 60.0	$ 60.0

Contractual interest rate for funds borrowed, under the debt agreement.

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