Derivatives (Details) (USD $)	12 Months Ended
	Dec. 31, 2012 derivatives	Dec. 31, 2011	Dec. 31, 2010	Jun. 30, 2011 Treasury Locks [Member] derivatives	Sep. 30, 2007 Interest Rate Swaps [Member] derivatives	Jun. 30, 2004 Forward Swap [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number Of Interest Rate Derivatives Terminated				2	2
Notional amount of interest rate hedges terminated				$ 150,000,000	$ 100,000,000	$ 94,000,000
Treasury locks designated as cash flow hedges, fair value				5,300,000	3,260,000
Treasury locks designated as cash flow hedges, other comprehensive income				5,218,000	3,228,000	4,148,000
Fair value of interest rate hedges recognized in other comprehensive income	5,693,000
Interest rate hedges gain reclassified as reduction to interest expense	231,000	9,000	165,000
Interest rate hedges gain (loss) to be reclassified into interest expense over next 12 months	$ 249,000
Number of Interest Rate Derivatives Held	0