Debt and Capital Markets (Schedule of Long-term Debt Instruments) (Details) $ in Thousands					3 Months Ended		6 Months Ended	9 Months Ended
	Oct. 31, 2017 swap_agreements	Sep. 27, 2017 USD ($)	Apr. 06, 2017 shares	Apr. 05, 2017 USD ($)	Sep. 30, 2017 USD ($) loans properties shares	Sep. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) loans properties shares	Sep. 30, 2016 USD ($)	May 23, 2017 USD ($)	Dec. 31, 2016 USD ($) shares
Debt Instrument [Line Items]
Extinguishment of Debt, Amount					$ 73,000
Number Of Mortgage Notes Receivable | loans					9			9
Number of Properties Securing Mortgage Note | properties					9			9
Common Shares, shares issued | shares					76,397,669			76,397,669			66,263,487
Net proceeds from issuance of common shares								$ 68,552	$ 142,452
Gain on Extinguishment of Debt					$ 0	$ 0		977	0
Long-term Debt					2,987,925			2,987,925			$ 2,485,625
Costs associated with loan refinancing or payoff					$ 1,477	$ 14		$ 1,491	$ 905
direct share purchase plan [Member]
Debt Instrument [Line Items]
Common Shares, shares issued | shares					928,219			928,219
Net proceeds from issuance of common shares								$ 67,900
Common Stock [Member]
Debt Instrument [Line Items]
Stock Issued During Period, Shares, Acquisitions | shares			8,851,264					8,851,264
Combined unsecured revolving credit and term loan facility [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 2,400,000
Line of Credit Facility, Current Borrowing Capacity		1,400,000
Mortgage note, due July 6, 2017 [Member]
Debt Instrument [Line Items]
Extinguishment of Debt, Amount					$ 87,000
Number of Properties Securing Mortgage Note | properties					11			11
Gain on Extinguishment of Debt				$ 1,000
Senior Unsecured Notes Payable, 4.50 Percent, Due June 1, 2027 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount										$ 450,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate					4.50%			4.50%
Senior Unsecured Notes Payable, Percent of Principal Amount Issued					0.99393			0.99393
Debt Covenant, Debt to Adjusted Total Assets Ratio, Maximum								0.6
Debt Covenant, Secured Debt to Adjusted Total Assets Ratio, Maximum								0.4
Debt Covenant, Debt Service Coverage Ratio, Minimum								1.5
Debt Covenant, Total Unencumbered Assets to Outstanding Unsecured Debt Ratio, Minimum								150.00%
Bond Payable, Variable Rate, Due October 1,2037 [Member] | Corporate Debt Securities [Member]
Debt Instrument [Line Items]
Long-term Debt					$ 25,000			$ 25,000
Unsecured revolving credit facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity		$ 1,000,000					$ 650,000
Debt Instrument, Basis Spread on Variable Rate		1.00%					1.25%
Line of Credit Commitment Fee Percentage		0.20%					0.25%
Costs associated with loan refinancing or payoff								19
Line of Credit Facility, Fair Value of Amount Outstanding					170,000			170,000
Unsecured term loan [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 400,000
Line of Credit Facility, Current Borrowing Capacity		$ 400,000					$ 350,000
Debt Instrument, Basis Spread on Variable Rate		1.10%					1.40%
Costs associated with loan refinancing or payoff								1,500
Line of Credit Facility, Fair Value of Amount Outstanding					50,000			50,000
Unsecured Revolving Variable Rate Credit Facility, Variable Rate, Due April 24,2019 [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity					$ 1,000,000			$ 1,000,000
Senior Unsecured Notes Payable, 4.75% due December 15, 2026 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate					4.75%			4.75%
Senior Unsecured Notes Payable, 4.50 Percent, Due April 1, 2025 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate					4.50%			4.50%
Senior unsecured notes payable, 5.25 Percent, Due July 15, 2023 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate					5.25%			5.25%
Senior Unsecured Notes Payable, 5.75 Percent, Due August 15, 2022 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate					5.75%			5.75%
Senoir Unsecured Notes Payable, 7.75 Percent, Due July 15, 2020 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate					7.75%			7.75%
senior unsecured private placement notes payable, 4.35 percent, due August 22, 2024 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate					4.35%			4.35%
senior unsecured private placement notes payable, 4.56 percent, due August 22, 2026 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate					4.56%			4.56%
interest rate swap 3.15percent [Member] | Subsequent Event [Member] | Interest Rate Swap [Member]
Debt Instrument [Line Items]
Number of entered into interest rate swap agreements (in interest rate swaps) | swap_agreements	3