Derivative Instruments (Details) - Interest rate contracts $ in Millions	3 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($)	Mar. 31, 2015 agreement	Sep. 30, 2015 USD ($)	Dec. 31, 2014 USD ($) agreement	Dec. 31, 2013 USD ($) agreement
Derivative instruments
Variable rate of debt		one-month LIBOR		one-month LIBOR
Borrowings at variable interest rates	$ 530.4		$ 530.4	$ 468.5
Number of interest rate swap agreements | agreement		0		0
Number of interest rate swap agreements during the period | agreement					1
Notional amount outstanding					$ 100.0
Fixed interest rate (as a percent)					2.10%
(Benefit) expense recorded to net finance costs	$ 0.1		$ 0.4