Fair Value (Tables)
 9 Months Ended
Sep. 30, 2016
Fair Value Disclosures [Abstract]  
Carrying Amount And Estimated Fair Values Of Financial Instruments
 
(in millions)
Carrying
Amount as of
September 30, 2016
 
Estimated Fair
Value as of
September 30, 2016
 
Carrying
Amount as of
Dec. 31, 2015
 
Estimated Fair
Value as of
Dec. 31, 2015
Long-term debt-affiliated
$
630.9

 
$
683.2

 
$
630.9

 
$
630.9