Derivative Financial Instruments (Details)	3 Months Ended						3 Months Ended
	Mar. 31, 2013 USD ($)	Mar. 31, 2013 Interest rate swap, variable rate, secured debt Interest rate swap, entered in July 2005, maturity in July 2020 USD ($) item	Mar. 31, 2013 Interest rate swap, variable rate, secured debt Interest rate swap, entered in November 2008, maturity in October 2016 USD ($)	Mar. 31, 2013 Interest rate swap, variable rate, secured debt Interest rate swap, entered in July 2009, maturity in July 2013 USD ($)	Mar. 31, 2013 Interest rate swap, variable rate, secured debt Interest rate swap, entered in July 2012, maturity in June 2016 USD ($)	Mar. 31, 2013 Interest rate swap, variable rate, secured debt Interest rate swap, entered in July 2012, maturity in June 2016 GBP (£)	Mar. 31, 2013 Foreign currency swap contract Foreign currency swap, entered in July 2012, maturity in June 2016 USD ($) item	Mar. 31, 2013 Foreign currency swap contract Foreign currency swap, entered in July 2012, maturity in June 2016 GBP (£)
Derivative
Fixed Rate/Buy Amount (as a percent)		3.82%	5.95%	6.13%	1.81%
Floating/Exchange Rate Index		BMA Swap Index	1 Month LIBOR	1 Month LIBOR	1 Month GBP LIBOR
Floating/Exchange Rate Index, percentage			1.50%	3.65%	1.20%
Notional / Sell Amount		$ 45,600,000	$ 26,900,000	$ 13,600,000		£ 137,000,000
Notional amount of foreign currency derivative purchase contracts							79,600,000
Notional amount of foreign currency derivative sale contracts								50,700,000
Semi-annual buy (sell) amount							11,400,000	(7,200,000)
Fair value of hedge, liabilities		(8,189,000)	(3,603,000)	(79,000)	(549,000)
Number of interest-rate swap contracts		3
Exchange rate USD/GBP							1.5695
Fair value of hedge, assets							2,490,000
Gain (loss) on cash flow hedge, ineffective portion					0		0
Gains or losses recorded to accumulated other comprehensive loss reclassified to earnings	0
Effects of Change in Interest Rates
+50 Basis Points		1,623,000	464,000	16,000	3,367,000		(336,000)
-50 Basis Points		(1,520,000)	(455,000)	(20,000)	(3,263,000)		434,000
+100 Basis Points		3,195,000	924,000	34,000	6,683,000		(721,000)
-100 Basis Points		$ (3,092,000)	$ (915,000)	$ (38,000)	$ (6,578,000)		$ 820,000
Number of foreign exchange contracts							7