Debt (Credit Facilities Narrative) (Details)	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Jan. 01, 2016
Revolving Line of Credit [Abstract]
Outstanding borrowings on term loans	$ 550,000,000	$ 250,000,000
Line of Credit	27,000,000
Line of credit facility covenant minimum tangible net worth	$ 900,000,000
Line of credit facility covenant percentage of net cash proceeds of issuance and sales of equity interests (in hundredths)	75.00%
Line of credit, financial covenant terms	The credit agreement providing for the $500,000 unsecured credit facility and $300,000 unsecured term loan include certain financial covenants and requires that we maintain: (1) a minimum tangible net worth of $900,000, plus an amount equal to 75% of the net cash proceeds of all issuances and primary sales of equity interests of the parent guarantor or any of its subsidiaries consummated following the closing date; (2) annual distributions not to exceed 95% of adjusted funds from operations; and (3) certain financial ratios, including the following:· a fixed charge coverage ratio of not less than 1.45 to 1.00, which increases to 1.50 to 1.00 as of January 1, 2016;· a maximum leverage ratio of not more than 60%; and· a maximum secured debt leverage ratio of 50%, which decreases to 45% as of January 1, 2016
Line of credit facility covenant maximum annual distributions (in hundredths)	95.00%
Line of credit facility covenant fixed charge coverage ratio	1.45
Line of credit facility covenant maximum leverage ratio (in hundredths)	60.00%
Scenario, Forecast [Member]
Revolving Line of Credit [Abstract]
Line of credit facility covenant fixed charge coverage ratio				1.50
Minimum [Member]
Revolving Line of Credit [Abstract]
Line of credit facility covenant maximum secured debt leverage ratio (in hundredths)	45.00%
Maximum [Member]
Revolving Line of Credit [Abstract]
Line of credit facility covenant maximum secured debt leverage ratio (in hundredths)	50.00%
$300 Million Senior Unsecured Term Loan Agreement [Member]
Revolving Line of Credit [Abstract]
Debt instrument, face amount	$ 300,000,000
Maturity date	Aug. 10, 2020
Outstanding borrowings on term loans	$ 300,000,000
$300 Million Senior Unsecured Term Loan Agreement [Member] | Minimum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)	1.50%
$300 Million Senior Unsecured Term Loan Agreement [Member] | Maximum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)	2.25%
$500 Million Senior Unsecured Credit Agreement [Member]
Revolving Line of Credit [Abstract]
Debt instrument, face amount		$ 500,000,000
Revolving line of credit, maximum borrowing capacity		$ 850,000,000
Line of credit, expiration date		Feb. 28, 2018
Renewal period of line of credit		1 year
$400 Million Senior Unsecured Credit Agreement [Member]
Revolving Line of Credit [Abstract]
Revolving line of credit, current borrowing capacity		$ 400,000,000
$400 Million Senior Unsecured Credit Agreement [Member] | Minimum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)		1.75%
$400 Million Senior Unsecured Credit Agreement [Member] | Maximum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)		2.65%
Senior Unsecured Borrowing Capacity [Member]
Revolving Line of Credit [Abstract]
Revolving line of credit, current borrowing capacity	$ 800,000,000
$250 Million Senior Unsecured Revolving Line Of Credit [Member]
Revolving Line of Credit [Abstract]
Revolving line of credit, current borrowing capacity		$ 250,000,000
Line of Credit	27,000,000	$ 0
$250 Million Senior Unsecured Revolving Line Of Credit [Member] | Minimum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)		1.70%
$250 Million Senior Unsecured Revolving Line Of Credit [Member] | Maximum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)		2.45%
$250 Million Unsecured Term Loan [Member]
Revolving Line of Credit [Abstract]
Debt instrument, face amount		$ 250,000,000
Outstanding borrowings on term loans	250,000,000	$ 250,000,000
Notional amount	$ 150,000,000
Effective fixed interest rate	2.914%
$250 Million Unsecured Term Loan [Member] | Minimum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)		1.60%
$250 Million Unsecured Term Loan [Member] | Maximum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)		2.35%
$150 Million Unsecured Term Loan [Member]
Revolving Line of Credit [Abstract]
Revolving line of credit, current borrowing capacity		$ 150,000,000
Revolving Line Of Credit [Member]
Revolving Line of Credit [Abstract]
Revolving line of credit, current borrowing capacity		250,000,000
Interest Expense, on credit facilities	$ 10,147,000	$ 6,218,000	$ 5,413,000
Line of credit, weighted average interest rate (in hundredths)	2.69%	2.82%	3.08%
$500 Million Senior Unsecured Credit Agreement And $300 Million Senior Unsecured Term Loan Agreement [Member]
Revolving Line of Credit [Abstract]
Line of credit, remaining borrowing capacity	$ 218,745,000