Summary of Significant Accounting Policies - Components of rental income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Property operating income	$ 363,907	$ 336,409	$ 306,550
Single-tenant net lease	52,873	63,871	67,238
Straight-line rent	6,072	7,201	9,545
Rental income	$ 422,852	$ 407,481	$ 383,333