Derivatives (Details) (USD $)	12 Months Ended							12 Months Ended
	Dec. 31, 2013 instrument	Dec. 31, 2012	Dec. 31, 2011	Jun. 30, 2011 Treasury Locks instrument	Sep. 30, 2007 Interest Rate Swap instrument	Apr. 30, 2013 Forward Swap instrument	Jun. 30, 2004 Forward Swap	Dec. 31, 2013 Reclassification out of Accumulated Other Comprehensive Income	Dec. 31, 2012 Reclassification out of Accumulated Other Comprehensive Income	Dec. 31, 2011 Reclassification out of Accumulated Other Comprehensive Income
Schedule of Trading Securities and Other Trading Assets [Line Items]
Number Of Interest Rate Derivatives Terminated				2	2	4
Notional amount of interest rate hedges terminated				$ 150,000,000	$ 100,000,000	$ 240,000,000	$ 94,000,000
Treasury locks designated as cash flow hedges, fair value				5,300,000	3,260,000	3,156,000
Treasury locks designated as cash flow hedges, other comprehensive income				5,218,000	3,228,000	3,141,000	4,148,000
Fair value of interest rate hedges recognized in other comprehensive income	8,396,000
Interest expense	85,283,000	83,192,000	75,532,000					438,000	231,000	9,000
Interest rate hedges gain (loss) to be reclassified into interest expense over next 12 months	$ 849,000
Number of Interest Rate Derivatives Held	0