Derivative Instruments (Details) (USD $)	12 Months Ended																	12 Months Ended
	Dec. 31, 2013 agreement	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Interest Rate Contract [Member] Interest Expense [Member]	Dec. 31, 2012 Interest Rate Contract [Member] Interest Expense [Member]	Dec. 31, 2011 Interest Rate Contract [Member] Interest Expense [Member]	Dec. 31, 2013 Interest Rate Swap [Member]	Dec. 31, 2013 Interest Rate Swap 1 [Member]	Dec. 31, 2013 Interest Rate Swap 2 [Member]	Dec. 31, 2013 Interest Rate Swap 3 [Member]	Dec. 31, 2013 Interest Rate Swap 4 [Member]	Dec. 31, 2013 Forward Starting Interest Rate Swap 3 [Member]	Dec. 31, 2013 Forward Starting Interest Rate Swap 4 [Member]	Dec. 31, 2013 Forward Starting Interest Rate Swap 5 [Member]	Dec. 31, 2013 Forward Starting Interest Rate Swap 6 [Member]	Dec. 31, 2013 Credit Risk Contract [Member]	Dec. 31, 2013 Debt [Member]	Dec. 31, 2013 Senior Notes [Member] $350 Million Senior Notes [Member] agreement	Dec. 31, 2013 Senior Notes [Member] $350 Million Senior Notes [Member] Forward Starting Interest Rate Swap [Member]	Dec. 31, 2013 Unsecured Debt [Member] $300 Million 2011 Unsecured Term Loan Maturing 2016 [Member] agreement	Dec. 31, 2013 Unsecured Debt [Member] $350 Million Senior Notes [Member]	May 09, 2013 Unsecured Debt [Member] $350 Million Senior Notes [Member]	Dec. 31, 2013 Unsecured Debt [Member] Unsecured 300 million 2013 Term Loan Maturing 2019 [Member]	Dec. 18, 2013 Unsecured Debt [Member] Unsecured 300 million 2013 Term Loan Maturing 2019 [Member]	Jan. 31, 2014 Unsecured Debt [Member] Unsecured 300 million 2013 Term Loan Maturing 2019 [Member] Subsequent Event [Member] agreement	Jan. 31, 2014 Unsecured Debt [Member] Unsecured 300 million 2013 Term Loan Maturing 2019 [Member] Interest Rate Swap [Member] Subsequent Event [Member]
Derivative [Line Items]
Number of interest rate swap agreements																				4					4
Total notional amount of forward starting interest rate agreements	$ 530,000,000						$ 580,000,000	$ 125,000,000	$ 75,000,000	$ 50,000,000	$ 50,000,000	$ 70,000,000	$ 70,000,000	$ 70,000,000	$ 70,000,000				$ 250,000,000	$ 300,000,000						$ 200,000,000
Debt instrument, face amount																				300,000,000	350,000,000	350,000,000	300,000,000	300,000,000
Number of forward starting interest rate swap agreements	6
Number of forward starting swap agreements settled during the period																		2
Gain on settlement of forward starting swap agreements	(672,000)	0	0															700,000
Term of notes																		10 years
Hedged amount for potential issuance of long term debt																	280,000,000
Derivative, term of contract	123 months
Interest Rate Cash Flow Hedge Derivative at Fair Value, Net [Abstract]
Interest rate swaps	24,176,000	1,075,000
Interest rate swaps	(4,526,000)	(8,235,000)
Net derivative asset/(liability)	19,650,000	(7,160,000)
Amount of loss/(gain) recognized in OCI on derivatives				(24,312,000)	7,656,000	3,064,000
Amount of previously recorded loss reclassified from accumulated OCI into interest expense				(3,126,000)	(3,033,000)	(1,218,000)
Amount that will be reclassified from accumulated other comprehensive income over the next twelve months	(7,300,000)
Assets needed for immediate settlement, aggregate fair value																$ 4,700,000