Derivative Instruments (Narrative) (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2016 USD ($) Derivative	Feb. 29, 2016 USD ($)	Dec. 31, 2015 USD ($)
Derivatives, Fair Value [Line Items]
Term loan debt	[1]	$ 586,756		$ 606,337
Net losses expected to be reclassified into earnings in the next twelve months		200
Term Loans [Member]
Derivatives, Fair Value [Line Items]
Term loan debt		$ 349,500	$ 27,500	$ 322,000
Interest Rate Swaps [Member]
Derivatives, Fair Value [Line Items]
Number of interest rate swap agreements | Derivative		5
Notional amounts		$ 69,300
Medium-term notes		19,300
Senior Notes		$ 50,000
Interest Rate Swaps [Member] | Cash Flow Hedging [Member]
Derivatives, Fair Value [Line Items]
Number of interest rate swap agreements | Derivative		1
Notional amounts		$ 27,500
Swaps fixed interest rate		2.15%
LIBOR variable interest rate		3.88%
Interest Rate Swaps [Member] | Cash Flow Hedging [Member] | Term Loans [Member]
Derivatives, Fair Value [Line Items]
Term loan debt		$ 27,500
Derivative, termination year		2026-02
Interest Rate Swaps [Member] | Minimum [Member]
Derivatives, Fair Value [Line Items]
Swaps fixed interest rate		7.50%
LIBOR variable interest rate		5.84%
Interest Rate Swaps [Member] | Maximum [Member]
Derivatives, Fair Value [Line Items]
Swaps fixed interest rate		8.89%
LIBOR variable interest rate		6.29%

[1] The carrying amount of long-term debt includes principal and unamortized discounts.