Rate Regulation - Regulatory Liabilities (Details) - USD ($) $ in Millions		Sep. 30, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Regulatory liabilities, current		$ 11.6	$ 4.0
Regulatory liabilities, noncurrent		186.4	65.6	[1]
Purchased-gas adjustment
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	[2]	2.6	0.0
Regulatory liabilities, noncurrent	[2]	0.0	0.0
Cost of plant removal
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	[3]	8.3	3.7
Regulatory liabilities, noncurrent	[3]	185.1	65.5
Income taxes refundable to customers
Regulatory Liabilities [Line Items]
Regulatory liabilities, current	[4]	0.0	0.0
Regulatory liabilities, noncurrent	[4]	0.1	0.1
Other
Regulatory Liabilities [Line Items]
Regulatory liabilities, current		0.7	0.3
Regulatory liabilities, noncurrent		$ 1.2	$ 0.0

[1] Derived from 2015 audited financial statements

[2] Purchased-gas costs that are different from those provided for in present rates are accumulated and recovered or credited through future rate changes.

[3] Cost of plant removal and AROs represent amounts recovered from customers for costs of future activities to remove assets that are expected to be incurred at the time of retirement.

[4] Income taxes refundable to customers arise from adjustments to deferred taxes, refunded over the life of the related property, plant and equipment.