Debt Commitments - Debt Obligations and Letters of Credit Outstanding (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Total debt	$ 500.0	$ 500.0
Less: Current portion of debt	(3.0)	(3.0)
Long-term debt	497.0	497.0
Credit Facility \| Unsecured Revolving Credit Facility
Debt Instrument [Line Items]
Total debt	365.0	365.0
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]
Total debt	$ 40.0	40.0
Fixed interest rate per annum	2.00%
Senior Unsecured Promissory Notes \| 2.45% Senior Unsecured Promissory Note Payable
Debt Instrument [Line Items]
Total debt	$ 35.0	35.0
Fixed interest rate per annum	2.45%
Senior Unsecured Promissory Notes \| 3.22% Senior Unsecured Promissory Note Payable
Debt Instrument [Line Items]
Total debt	$ 60.0	$ 60.0
Fixed interest rate per annum	3.22%

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

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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