Goodwill and Asset Impairments (Tables)

12 Months Ended

Dec. 31, 2015

Goodwill And Intangible Assets Disclosure [Abstract]

Changes in Carrying Amount of Goodwill

The following table presents the changes in the carrying amount of goodwill for the years ended Dec. 31, 2015, 2014 and 2013.

(millions)	NMGC		Total
Balance as of Dec. 31, 2013	$	0.0	$	0.0
Acquisition of NMGC		408.3		408.3
Balance as of Dec. 31, 2014		408.3		408.3
Measurement period adjustments (1)		0.1		0.1
Balance as of Dec. 31, 2015	$	408.4	$	408.4

(1) Due to immateriality, the measurement period adjustment was not applied retrospectively to the opening balance sheet.