Summary of Significant Accounting Policies - Components of rental income (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Accounting Policies [Abstract]
Property operating income	$ 256,129	$ 239,707	$ 216,058
Single-tenant net lease	52,665	42,481	41,957
Straight-line rent	9,500	6,599	5,725
Rental income	$ 318,294	$ 288,787	$ 263,740