Investment Property (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Property Subject To Or Available For Operating Lease [Line Items]
Investment property, net	$ 2,707,565		$ 2,689,276
Sale consideration of sold property			1,000,000
Gain on sale of real estate investments	19	$ 85,188	364,300
Building and improvements
Property Subject To Or Available For Operating Lease [Line Items]
Investment property, gross	2,288,189		2,255,267
Less: accumulated depreciation	(253,543)		(237,767)
Investment property, net	2,034,646		2,017,500
Land
Property Subject To Or Available For Operating Lease [Line Items]
Investment property, gross	$ 672,919		$ 671,776