Derivative Financial Instruments (Details)	3 Months Ended	9 Months Ended					9 Months Ended
	Mar. 31, 2011 Interest rate swap, entered in August 2009, maturity in August 2011 USD ($)	Sep. 30, 2012 Interest rate swap, variable rate, secured debt Interest rate swap, entered in July 2005, maturity in July 2020 USD ($) item	Sep. 30, 2012 Interest rate swap, variable rate, secured debt Interest rate swap, entered in November 2008, maturity in October 2016 USD ($)	Sep. 30, 2012 Interest rate swap, variable rate, secured debt Interest rate swap, entered in July 2009, maturity in July 2013 USD ($)	Sep. 30, 2012 Interest rate swap, variable rate, secured debt Interest rate swap, entered in July 2012, maturity in June 2016 USD ($)	Sep. 30, 2012 Interest rate swap, variable rate, secured debt Interest rate swap, entered in July 2012, maturity in June 2016 GBP (£)	Sep. 30, 2012 Foreign currency swap contract Foreign currency swap, entered in July 2012, maturity in June 2016 USD ($)
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	$ 27,200,000	$ 13,700,000		£ 137,000,000
Notional amount of foreign currency derivative purchase contracts							11,400,000
Notional amount of foreign currency derivative sale contracts							7,200,000
Fair value of hedge, liabilities		(8,868,000)	(4,142,000)	(234,000)	(75,000)		(2,317,000)
Exchange rate USD/GBP							1.5695
Number of interest-rate swap contracts		3
Reclassification of unrealized gains into other income	1,000,000
Proceeds from settlement of interest rate swap contract	1,000,000
Effects of Change in Interest Rates
+50 Basis Points		1,605,000	528,000	49,000	4,238,000		(699,000)
-50 Basis Points		(1,760,000)	(526,000)	(55,000)	(3,872,000)		232,000
+100 Basis Points		3,287,000	1,055,000	102,000	8,293,000		(1,164,000)
-100 Basis Points		$ (3,443,000)	$ (1,052,000)	$ (107,000)	$ (7,926,000)		$ 697,000