Debt (Credit Facilities Narrative) (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) agreement	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Feb. 21, 2017 USD ($) item
Revolving Line of Credit [Abstract]
Long-term debt	$ 1,110,438,000
Outstanding borrowings on term loans	663,500,000	$ 547,780,000
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 Facility and the Term Loans include certain financial covenants and require that we maintain: (1) a minimum tangible net worth (calculated as total assets, plus accumulated depreciation, less total liabilities, intangibles and other defined adjustments) 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 1.50 to 1.00, · a maximum leverage ratio of not more than 60%; and· a maximum secured debt leverage ratio of 45%
Line of credit facility covenant maximum annual distributions (in hundredths)	95.00%
Line of credit facility covenant fixed charge coverage ratio	1.50
Line of credit facility covenant maximum leverage ratio (in hundredths)	60.00%
Maximum [Member]
Revolving Line of Credit [Abstract]
Line of credit facility covenant maximum secured debt leverage ratio (in hundredths)	45.00%
Unsecured Credit Agreements [Member]
Revolving Line of Credit [Abstract]
Number of unsecured credit agreements | agreement	3
Revolving line of credit, current borrowing capacity	$ 1,000,000,000
$500 Million Senior Unsecured Credit Agreement [Member]
Revolving Line of Credit [Abstract]
Maturity date	Feb. 28, 2018
Revolving line of credit, current borrowing capacity	$ 500,000,000
Revolving line of credit, maximum borrowing capacity	$ 850,000,000
Renewal period of line of credit	1 year
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	$ 17,332,000	$ 10,147,000	$ 6,218,000
Line of credit, weighted average interest rate (in hundredths)	2.82%	2.69%	2.82%
$250 Million Unsecured Term Loan [Member]
Revolving Line of Credit [Abstract]
Debt instrument, face amount	$ 250,000,000
$250 Million Unsecured Term Loan [Member] | Interest Rate Swap [Member]
Revolving Line of Credit [Abstract]
Fixed interest rate	2.914%
Notional amount	$ 150,000,000
$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%
$300 Million Senior Unsecured Term Loan Agreement ("Second Term Loan") [Member]
Revolving Line of Credit [Abstract]
Debt instrument, face amount	$ 300,000,000
Maturity date	Aug. 10, 2020
$300 Million Senior Unsecured Term Loan Agreement ("Second Term Loan") [Member] | Interest Rate Swap [Member]
Revolving Line of Credit [Abstract]
Fixed interest rate	3.211%
$300 Million Senior Unsecured Term Loan Agreement ("Second Term Loan") [Member] | Minimum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)	1.50%
$300 Million Senior Unsecured Term Loan Agreement ("Second Term Loan") [Member] | Maximum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)	2.25%
$200 Million Senior Unsecured Term Loan Agreement ("Third Term Loan") [Member]
Revolving Line of Credit [Abstract]
Debt instrument, face amount	$ 200,000,000
Maturity date	Aug. 02, 2021
$200 Million Senior Unsecured Term Loan Agreement ("Third Term Loan") [Member] | Interest Rate Swap [Member]
Revolving Line of Credit [Abstract]
Notional amount	$ 150,000,000
$200 Million Senior Unsecured Term Loan Agreement ("Third Term Loan") [Member] | Minimum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)	1.45%
$200 Million Senior Unsecured Term Loan Agreement ("Third Term Loan") [Member] | Maximum [Member]
Revolving Line of Credit [Abstract]
Basis spread on variable rate (in hundredths)	2.20%
Term Loans [Member]
Revolving Line of Credit [Abstract]
Deferred costs, net of accumulated amortization	$ 3,120,000	$ 2,220,000
$250 Million Senior Unsecured Revolving Line Of Credit [Member]
Revolving Line of Credit [Abstract]
Line of Credit		$ 27,000,000
$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%
$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	$ 99,822,000
Subsequent Event [Member] | $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				$ 197,998,000
Number of borrowing base assets added | item				7
Number of borrowing base assets removed | item				2