Notes Payable - Convertible (Details) (USD $)	3 Months Ended			1 Months Ended	3 Months Ended		12 Months Ended	3 Months Ended		12 Months Ended			3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Jan. 31, 2013 2026 Notes [Member]	Mar. 31, 2013 2026 Notes [Member]		Dec. 31, 2012 2026 Notes [Member]	Mar. 31, 2013 2028 Notes [Member]		Dec. 31, 2012 2028 Notes [Member]	Dec. 31, 2009 2028 Notes [Member]		Mar. 31, 2013 Convertible Debt [Member]	Mar. 31, 2012 Convertible Debt [Member]	Dec. 31, 2012 Convertible Debt [Member]
Debt Instrument [Line Items]
Proceeds from Convertible Debt					$ 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]
Principal amount of debt					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]		0	[1],[3],[4]
Outstanding principal balance	223,035,000		238,572,000		0	[1],[2],[7]	15,537,000	223,035,000	[1],[3],[4]
Conversion rate basis							1	1
Convertible notes payable, shares issued upon conversion							42.6237	39.5149
Conversion price							$ 23.4611	$ 25.3069
Repayments of convertible debt	20,565,000	0		20,565,000							8,588,000
Gains (losses) on extinguishment of debt											1,867,000
Write off of deferred debt issuance cost											219,000
Convertible, effective interest rate					5.84%			7.192%
Carrying value of equity component	(20,720,000)		(22,193,000)
Convertible notes payable unamortized discount	(951,000)		(2,072,000)
Net carrying amount of convertible debt	201,364,000		214,307,000
Remaining discount amortization period								3 years
Noncash interest charges	1,356,000	1,181,000											1,122,000	1,044,000
Contractual interest expense													2,858,000	4,227,000
Amortization of debt costs													305,000		0
Total interest expense													4,285,000	5,271,000
Convertible, If-converted value in rxcess of principal							5,125,000	95,739,000		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 have been deferred and are being amortized over the period to the earliest put option date of the holders using the effective interest method.

[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] The conversion rate per $1 principal amount was 39.5149 shares of NNN’s common stock, which is equivalent to a conversion price of $25.3069 per share of common stock.

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

[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.

[7] In January 2013, NNN settled the remaining principal balance of $15,537.