Borrowings Under Lines of Credit (Details)	9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Line of Credit Facility [Line Items]
Borrowings under line of credit	$ 75,000,000	$ 90,000,000
Repayments of lines of credit	200,000,000	$ 90,000,000
KWH Facility [Member] | Unsecured Debt [Member] | U.S. Bank, East-West Bank and Bank of Ireland [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 300,000,000.0
Rent adjusted fixed charge coverage ratio	2.57
Rolling average basis period	12 months
Balance sheet leverage ratio	0.67
Covenant, tangible net worth	$ 1,051,100,000
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	592,900,000
Borrowings under line of credit	75,000,000
Repayments of lines of credit	200,000,000
Maximum amount drawn on the unsecured credit facility at any one point	150,000,000
Remaining borrowing capacity	$ 300,000,000		$ 175,000,000
Outstanding unsecured facility line of credit			$ 125,000,000
KWH Facility [Member] | Unsecured Debt [Member] | U.S. Bank, East-West Bank and Bank of Ireland [Member] | Revolving Credit Facility [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Rent adjusted fixed charge coverage ratio	1.50
Covenant, tangible net worth	$ 500,000,000.0
KWH Facility [Member] | Unsecured Debt [Member] | U.S. Bank, East-West Bank and Bank of Ireland [Member] | Revolving Credit Facility [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Balance sheet leverage ratio	1.50
KWH Facility [Member] | Unsecured Debt [Member] | U.S. Bank, East-West Bank and Bank of Ireland [Member] | Revolving Credit Facility [Member] | LIBOR [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	2.75%