Derivative Instruments (Details) (USD $)	9 Months Ended		12 Months Ended
	Sep. 30, 2014 item	Sep. 30, 2013	Dec. 31, 2013 item	Nov. 30, 2013	Nov. 25, 2013
Derivative instruments
Variable rate of debt	one-month LIBOR
Interest Rate Contract [Member]
Derivative instruments
Variable rate of debt	one-month LIBOR	one-month LIBOR	one-month LIBOR
Borrowings at variable interest rates	$ 383,400,000	$ 383,400,000	$ 392,000,000		$ 0
Number of interest rate swap agreements	0		0
Number of interest rate swap agreements during the period			1
Notional amount outstanding		100,000,000		100,000,000
Fixed interest rate (as a percent)				2.10%
(Benefit) expense recorded to net finance costs	$ (400,000)	$ 1,100,000