Related Party Transactions - Schedule of Other Related Party Balances (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Related Party Transactions [Abstract]
Accounts receivable	$ 10.1	$ 9.8
Prepaid expenses	2.7	2.8
Accounts payable	5.2	9.3
Deferred revenue	7.9	3.6
Accrued liabilities	5.1	3.6
Debt payable	686.0	457.6
Lease liability	24.9	22.8
Accrued interest, related parties	2.6	1.3
Fuel accrual, related parties	1.6	1.2
Other accrued liabilities, related parties	0.9	0.5
FERC accrual, related parties		0.6
Unamortized debt issuance costs	$ 0.6	$ 0.9