Real Estate (Dispositions) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($) property	Dec. 31, 2015 USD ($) property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Income tax expense	$ 0	$ 0	$ (10,318)
Assets Held-for-sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold at a gain | property	48	38	19
Gain	$ 36,655	$ 27,182	$ 10,807
Income tax expense	0	0	(357)
Gain on disposition of assets, total, continuing and discontinued operations	$ 36,655	$ 27,182	$ 10,450