Line of Credit (Details)	9 Months Ended						9 Months Ended	1 Months Ended
	Sep. 30, 2014 USD ($)	Sep. 30, 2013 USD ($)	Sep. 30, 2014 KWF Facility [Member] Unsecured Debt [Member] U.S. Bank, East-West Bank and Bank of Ireland [Member] Revolving Credit Facility [Member] USD ($)	Jul. 31, 2014 KWF Facility [Member] Unsecured Debt [Member] U.S. Bank, East-West Bank and Bank of Ireland [Member] Revolving Credit Facility [Member] USD ($)	Jun. 30, 2014 KWF Facility [Member] Unsecured Debt [Member] U.S. Bank, East-West Bank and Bank of Ireland [Member] Revolving Credit Facility [Member] USD ($)	Sep. 30, 2014 KWF Facility [Member] Unsecured Debt [Member] U.S. Bank, East-West Bank and Bank of Ireland [Member] Revolving Credit Facility [Member] Minimum [Member] USD ($)	Sep. 30, 2014 KWF Facility [Member] Unsecured Debt [Member] U.S. Bank, East-West Bank and Bank of Ireland [Member] Revolving Credit Facility [Member] LIBOR [Member]	Sep. 30, 2014 KWE Facility [Member] Unsecured Debt [Member] Revolving Credit Facility [Member] USD ($)	Sep. 30, 2014 KWE Facility [Member] Unsecured Debt [Member] Revolving Credit Facility [Member] GBP (£)
Line of Credit Facility [Line Items]
Basis spread on variable rate							2.75%
Maximum borrowing capacity				$ 300,000,000.0	$ 140,000,000.0			$ 365,000,000	£ 225,000,000
Rent adjusted fixed charge coverage ratio			3.54			1.50
Balance sheet leverage ratio			0.79			1.50
Covenant, tangible net worth			897,600,000			500,000,000.0
Covenant, percent of new equity offerings			50.00%
Line of credit facility covenant compliance minimum requirement of unrestricted cash and cash equivalents						40,000,000.0
Cash and cash equivalents and marketable securities			800,600,000
Borrowings under line of credit	90,000,000	125,000,000	90,000,000
Maximum amount drawn on the unsecured credit facility at any one point			90,000,000
Remaining borrowing capacity			300,000,000.0
Debt instrument term								3 years	3 years
Covenant, maximum consolidated leverage ratio								60.00%	60.00%
Covenant, minimum net asset value									£ 744,400,000.0
Covenant, percent of cash proceeds received by subsidiaries								75.00%	75.00%
Covenant, fixed charge coverage ratio								1.5	1.5
Covenant, minimum unsecured interest								1.9	1.9
Covenant, maximum secured recourse indebtedness								2.50%	2.50%