Derivative Instruments (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Jul. 31, 2012	Dec. 31, 2011
Interest Rate Derivatives
Derivative reference rate	one-month floating rate LIBOR
Fixed interest rate (as a percent)	0.53875%	0.53875%
Variable rate debt swapped to fixed rate debt	$ 76,109	$ 300,000	$ 76,269
Interest rate swaps
Derivative Financial Instruments
Number of derivative instruments utilized to hedge variable cash flows	4		3
Gain (loss) on cash flow hedges expected to be reclassified to interest expense in the next year	1,987
Interest Rate Derivatives
Notional amount	300,000	300,000
Derivative reference rate	one-month floating rate LIBOR
Fixed interest rate (as a percent)	0.53875%
Variable rate debt swapped to fixed rate debt	300,000
Fair value	1,247
Number of instruments	4		3
Total notional amount	$ 376,109		$ 76,269