Derivatives Instrument and Hedging Activities (Details)	12 Months Ended
	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Derivatives Instrument and Hedging Activities
Interest paid	$ 34,837,000	$ 38,464,000	$ 40,414,000
Interest Rate Swap | Designated as Hedging Instrument | Cash Flow Hedging
Derivatives Instrument and Hedging Activities
Number of agreements | item	1
Notional amount	$ 200,000,000
Fixed interest rate (as a percent)	1.6775%
Interest paid	$ 547,000
Estimated amount to be reclassified to interest expense	$ 1,500,000