Notes Payable - Convertible (Details) (USD $)	6 Months Ended			1 Months Ended	6 Months Ended		12 Months Ended	6 Months Ended		12 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jan. 31, 2013 2026 Notes [Member]	Jun. 30, 2013 2026 Notes [Member]		Dec. 31, 2012 2026 Notes [Member]	Jun. 30, 2013 2028 Notes [Member]		Dec. 31, 2012 2028 Notes [Member]	Dec. 31, 2009 2028 Notes [Member]	Jun. 30, 2013 Convertible Debt [Member]	Jun. 30, 2012 Convertible Debt [Member]	Jun. 30, 2013 Convertible Debt [Member]	Jun. 30, 2012 Convertible Debt [Member]
Debt Instrument [Line Items]
Net Proceeds					$ 168,650,000	[1],[2]		$ 228,576,000	[1],[3],[4]
Stated interest rate					3.95%	[1],[2]		5.125%	[1],[3],[4]
Effective Interest Rate					5.84%	[1],[2],[5]		7.192%	[1],[3],[4],[5]
Debt Issuance Costs					3,850,000	[1],[2]		5,459,000	[1],[3],[4],[6]
Original Principal					172,500,000	[1],[2]		234,035,000	[1],[3],[4]
Repurchases					(33,800,000)	[1],[2]		(11,000,000)	[1],[3],[4]
Settled				(15,537,000)	(138,700,000)	[1],[2]		(91,000)	[1],[3],[4]
Outstanding principal balance	222,944,000		238,572,000		0	[1],[2]	15,537,000	222,944,000	[1],[3],[4]
Conversion rate basis					1			1
Convertible notes payable, shares issued upon conversion					42.6237			39.5360
Conversion price					$ 23.4611			$ 25.2934
Repayments of convertible debt	20,656,000	0		20,702,000							8,588,000
Gains (losses) on extinguishment of debt											1,867,000
Write off of deferred debt issuance cost								219,000
Principal amount of convertible debt	(20,711,000)		(22,193,000)
Remaining unamortized debt discount	0		(2,072,000)
Net carrying value of convertible debt	202,233,000		214,307,000
Noncash interest charges												2,516,000	4,227,000	5,374,000	8,455,000
Contractual interest expense	2,591,000	2,378,000										950,000	1,063,000	2,072,000	2,107,000
Amortization of debt costs												260,000	284,000	566,000	560,000
Total interest expense				137,000								3,726,000	5,574,000	8,012,000	11,122,000
Convertible, If-converted value in rxcess of principal							5,125,000	80,268,000		51,764,000
Decrease to additional paid-in capital				$ 5,028,000

[1] Debt issuance costs include underwriting discounts and commissions, legal and accounting fees, rating agency fees and printing expenses. These costs were deferred and amortized using the effective interest method and are fully amortized at June 30, 2013.

[2] The conversion rate per $1 principal amount was 42.6237 shares of NNN’s common stock, which is equivalent to a conversion price of $23.4611 per share of common stock.

[3] NNN repurchased $11,000 in 2009 for a purchase price of $8,588 resulting in a gain of $1,867.

[4] The conversion rate per $1 principal amount was 39.5360 shares of NNN’s common stock, which is equivalent to a conversion price of $25.2934 per share of common stock.

[5] With the adoption of the accounting guidance on convertible debt securities in 2009, the effective interest rates for the 2026 Notes and the 2028 Notes are 5.840% and 7.192%, respectively.

[6] Includes $219 of note costs which were written off in connection with the repurchase of $11,000 of the 2028 Notes.