Equity Investments - Summary of Balance Sheet Information of Equity Method Investee (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013	Dec. 31, 2012	Jul. 09, 2014 SIR	Dec. 31, 2013 SIR
Balance Sheets:
Real estate properties, net	$ 4,898,041	$ 4,642,106			$ 1,772,684	$ 1,579,234
Acquired real estate leases, net	212,584	255,812			125,163	129,426
Cash and cash equivalents	597,405	222,449	165,990	102,219	27,531	20,025
Rents receivable, net	238,207	223,769			60,474	55,335
Other assets, net	209,005	188,675			17,698	17,839
Total assets	6,170,796	6,646,434			2,003,550	1,801,859
Revolving credit facility	0	235,000			74,000	159,000
Term loan					350,000	350,000
Mortgage notes payable	619,760	914,510			19,069	27,147
Assumed real estate lease obligations, net	28,950	33,935			26,945	26,966
Other liabilities					44,225	40,055
Shareholders’ equity					1,489,311	1,198,691
Total liabilities and shareholders’ equity	$ 6,170,796	$ 6,646,434			$ 2,003,550	$ 1,801,859