Secured and Unsecured Debt (Details 5) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Aggregate maturities of unsecured debt
2013	$ 6,639
2014	359,562
2015	750,080
2016	568,925
2017	243,374
Thereafter	1,466,143
Unsecured Debt, Total	2,009,689	1,979,198
Long-term Debt	3,394,723	3,409,333
Secured Debt [Member]
Aggregate maturities of unsecured debt
2013	6,639
2014	47,984
2015	197,191
2016	136,440
2017	243,374
Thereafter	753,406
Long-term Debt	1,385,034
Unsecured Debt [Member]
Aggregate maturities of unsecured debt
2013	0
2014	311,578
2015	552,889
2016	432,485
2017	0
Thereafter	712,737
Unsecured Debt, Total	2,009,689	1,979,198
Long-term Debt	2,009,689
Variable Rate Debt [Member] \| Secured Debt [Member]
Aggregate maturities of unsecured debt
2013	0
2014	0
2015	0
2016	0
2017	65,000
Thereafter	241,109
Long-term Debt	306,109
Fixed Rate Debt [Member] \| Secured Debt [Member]
Aggregate maturities of unsecured debt
2013	6,639
2014	47,984
2015	197,191
2016	136,440
2017	178,374
Thereafter	512,297
Long-term Debt	$ 1,078,925

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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount of long-term debt, sinking fund requirements, and other securities redeemable at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year.

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