Derivatives (Narrative) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) instrument	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)	Oct. 31, 2015 USD ($)	May 31, 2014 USD ($)	Apr. 30, 2013 USD ($)
Derivative [Line Items]
Stockholders' equity	$ (4,222,451,000)	$ (3,989,111,000)	$ (4,222,451,000)	$ (3,989,111,000)	$ (4,152,887,000)	$ (4,154,605,000)		$ (3,864,506,000)	$ (3,840,910,000)
Interest expense	29,811,000	$ 27,110,000	59,768,000	53,712,000
Interest rate cash flow hedge gain (loss) to be reclassified over next 12 months, net	(1,333,000)		(1,333,000)
Fair value in other liabilities and accumulated other comprehensive income (loss)	$ 3,920,000		3,920,000
Forward Swap
Derivative [Line Items]
Number of derivatives entered | instrument	1
Notional amount	$ 90,000,000		90,000,000				$ 250,000,000			$ 250,000,000	$ 180,000,000	$ 300,000,000	$ 225,000,000	$ 240,000,000
Fair value in other liabilities and accumulated other comprehensive income (loss)							$ 4,080,000			$ (7,688,000)	$ 13,345,000	$ (13,369,000)	$ (6,312,000)	$ (3,141,000)
Reclassification out of other comprehensive income
Derivative [Line Items]
Interest expense			648,000	$ 1,056,000
AOCI, cash flow hedges
Derivative [Line Items]
Stockholders' equity	$ 6,263,000		$ 6,263,000