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

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Debt Commitments - Debt Obligations and Letters of Credit Outstanding (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Total debt	$ 445.0	$ 390.0
Less: Current portion of debt	(8.8)	(10.5)
Long-term debt	436.2	379.5
Credit Facility [Member] \| Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Total debt	305.0	305.0
Credit Facility [Member] \| Letter of Credit Subfacility [Member]
Debt Instrument [Line Items]
Outstanding letters of credit under unsecured revolving credit facility - contingent obligation	36.3	36.3
Senior Unsecured Promissory Notes [Member] \| 2.00% Senior Unsecured Promissory Note Payable [Member]
Debt Instrument [Line Items]
Total debt	$ 40.0	40.0
Fixed interest rate per annum	2.00%
Senior Unsecured Promissory Notes [Member] \| 2.45% Senior Unsecured Promissory Note Payable [Member]
Debt Instrument [Line Items]
Total debt	$ 35.0	35.0
Fixed interest rate per annum	2.45%
Senior Unsecured Promissory Notes [Member] \| 3.22% Senior Unsecured Promissory Note Payable [Member]
Debt Instrument [Line Items]
Total debt	$ 60.0	0.0
Fixed interest rate per annum	3.22%
Note Payable Under Asset Purchase Agreement [Member]
Debt Instrument [Line Items]
Total debt	$ 5.0	$ 10.0	$ 15.0
Fixed interest rate per annum	0.56%