Notes Payable - Convertible (Details) (USD $)	12 Months Ended					12 Months Ended					24 Months Ended		12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009	Sep. 30, 2010	Dec. 31, 2011 Two Thousand Twenty Six Notes Member		Dec. 31, 2010 Two Thousand Twenty Six Notes Member	Dec. 31, 2009 Two Thousand Twenty Six Notes Member	Dec. 31, 2008 Two Thousand Twenty Six Notes Member	Dec. 31, 2009 Two Thousand Twenty Six Notes Member		Dec. 31, 2011 2028 Notes [Member]		Dec. 31, 2010 2028 Notes [Member]	Dec. 31, 2009 2028 Notes [Member]		Dec. 31, 2011 2028 Notes [Member]
Debt Instrument [Line Items]
Proceeds from Convertible Debt						$ 168,650,000	[1],[2],[3]						$ 228,576,000	[1],[4],[5]
Debt Instrument, Interest Rate, Stated Percentage					8.50%	3.95%	[1],[2],[3],[6]						5.125%	[1],[4],[5],[6]
Deferred Finance Costs, Gross						3,850,000	[1],[2],[3],[7]						5,459,000	[1],[4],[5],[8]
Next Put Option For Holders Date	9/30/2016	[9]																6/30/2013
Debt Instrument, Face Amount						172,500,000	[1],[2],[3]						234,035,000	[1],[4],[5]
Repurchases of Convertible Debt									(8,800,000)	(25,000,000)	(33,800,000)	[1],[2],[3]				(11,000,000)	[1],[4],[5]
Long-term Debt, Gross	361,735,000		361,735,000			138,700,000	[1],[2],[3]						223,035,000	[1],[4],[5]
Repayments of Convertible Debt									6,994,000	19,188,000						8,588,000
Gains (Losses) on Extinguishment of Debt	0		0	3,432,000					1,565,000	4,961,000						1,867,000
Write off of Deferred Debt Issuance Cost											463,000					219,000
Debt Instrument, Convertible, Conversion Price						$ 23.6430							$ 25.3753
Convertible Notes Payable, Conversion Rate Basis						1							1.00
Convertible notes payable, shares issued upon conversion						42.2959							39.4084
Debt Instrument, Convertible, Carrying Amount of Equity Component	(33,873,000)		(33,873,000)
Convertible Notes Payable Unamortized Discount	(6,363,000)		(12,201,000)
Net carrying amount of convertible debt	321,499,000		315,661,000
Debt Instrument, Convertible, Remaining Discount Amortization Period													18
Other Noncash Expense	5,837,000		6,154,000	5,809,000
Debt Instrument, Convertible, Interest Expense	16,909,000		16,909,000	17,046,000
Debt Instrument, Convertible, If-converted Value in Excess of Principal						$ 16,057,000		$ 15,601,000					$ 8,831,000		$ 9,611,000
Debt Instrument, Convertible, Effective Interest Rate						5.84%							7.192%

[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] NNN repurchased $8,800 and $25,000 in 2009 and 2008, respectively, for a purchase price of $6,994 and $19,188, respectively, resulting in a gain of $1,565 and $4,961, respectively.

[3] The conversion rate per $1 principal amount was 42.2959 shares of NNN’s common stock, which is equivalent to a conversion price of $23.6430 per share of common stock.

[4] The conversion rate per $1 principal amount was 39.4084 shares of NNN’s common stock, which is equivalent to a conversion price of $25.3753 per share of common stock.

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

[6] 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.

[7] Includes $463 of note costs which were written off in connection with the repurchase of $33,800 of the 2026 Notes.

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

[9] None of the 2026 Notes were repurchased in connection with the prior put option date of September 2011.