Derivatives (Details)	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2020 USD ($) instrument	Sep. 30, 2018 USD ($) instrument	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) instrument	Dec. 31, 2016 USD ($) instrument	Oct. 31, 2015 USD ($) instrument	May 31, 2014 USD ($) instrument	Apr. 30, 2013 USD ($) instrument
Debt and Equity Securities, FV-NI [Line Items]
Stockholders' equity	$ (4,319,304,000)	$ (4,331,682,000)	$ (4,154,605,000)			$ (3,840,910,000)
Interest expense	129,431,000	120,023,000	115,847,000
Interest rate hedges loss to be reclassified into interest expense over next 12 months	2,597,000
Gain or Loss on Cash Flow Hedges
Debt and Equity Securities, FV-NI [Line Items]
Stockholders' equity	16,445,000	11,128,000	6,911,000
Reclassification out of Accumulated Other Comprehensive Income
Debt and Equity Securities, FV-NI [Line Items]
Interest expense	$ 2,300,000	$ 1,307,000	$ 3,664,000
Forward Swap
Debt and Equity Securities, FV-NI [Line Items]
Number of interest rate derivatives terminated | instrument				3	2		2	2	4	3	4
Aggregate Notional Amount				$ 200,000,000	$ 250,000,000		$ 250,000,000	$ 180,000,000	$ 300,000,000	$ 225,000,000	$ 240,000,000
Liability Fair Value When Terminated				13,141,000			7,690,000		13,369,000	6,312,000	3,156,000
Asset Fair Value When Terminated					4,080,000			13,352,000
Fair Value Deferred In Other Comprehensive Income				$ 13,141,000	$ (4,080,000)		$ 7,688,000	$ (13,345,000)	$ 13,369,000	$ 6,312,000	$ 3,141,000