Derivative Financial Instruments Outstanding Interest Rate Swap Agreements (Details) (USD $)	3 Months Ended
	Mar. 31, 2013
Derivatives [Line Items]
Gain/loss reclassified from accumulated other comprehensive into earnings, ineffective portion	$ 0
Interest rate swap agreements [Member]
Derivatives [Line Items]
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred	1,700,000
Interest Rate Swap 8 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Notional amount of interest rate derivatives	120,000,000
Derivative, fixed interest rate	3.88%
Interest Rate Swap 1 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Notional amount of interest rate derivatives	1,450,000,000
Derivative, fixed interest rate	0.25%
Interest Rate Swap 1 [Member] | Notional Period 2 [Member]
Derivatives [Line Items]
Notional amount of interest rate derivatives	1,350,000,000
Derivative, fixed interest rate	0.25%
Interest Rate Swap 1 [Member] | Notional Period 3 [Member]
Derivatives [Line Items]
Notional amount of interest rate derivatives	1,300,000,000
Derivative, fixed interest rate	0.25%
Interest Rate Swap 1 [Member] | Notional Period 4 [Member]
Derivatives [Line Items]
Notional amount of interest rate derivatives	1,250,000,000
Derivative, fixed interest rate	0.25%
Interest Rate Swap 2 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Notional amount of interest rate derivatives	1,350,000,000
Derivative, fixed interest rate	0.35%
Interest Rate Swap 2 [Member] | Notional Period 2 [Member]
Derivatives [Line Items]
Notional amount of interest rate derivatives	1,300,000,000
Derivative, fixed interest rate	0.35%
Interest Rate Swap 2 [Member] | Notional Period 3 [Member]
Derivatives [Line Items]
Notional amount of interest rate derivatives	1,250,000,000
Derivative, fixed interest rate	0.35%
Interest Rate Swap 2 [Member] | Notional Period 4 [Member]
Derivatives [Line Items]
Notional amount of interest rate derivatives	1,200,000,000
Derivative, fixed interest rate	0.35%
Interest Rate Swap 3 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Notional amount of interest rate derivatives	1,150,000,000
Derivative, fixed interest rate	0.54%
Interest Rate Swap 3 [Member] | Notional Period 2 [Member]
Derivatives [Line Items]
Notional amount of interest rate derivatives	1,100,000,000
Derivative, fixed interest rate	0.54%
Interest Rate Swap 3 [Member] | Notional Period 3 [Member]
Derivatives [Line Items]
Notional amount of interest rate derivatives	1,050,000,000
Derivative, fixed interest rate	0.54%
Interest Rate Swap 3 [Member] | Notional Period 4 [Member]
Derivatives [Line Items]
Notional amount of interest rate derivatives	1,000,000,000
Derivative, fixed interest rate	0.54%
Interest Rate Swap 4 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Notional amount of interest rate derivatives	350,000,000
Derivative, fixed interest rate	0.93%
Interest Rate Swap 5 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Notional amount of interest rate derivatives	100,000,000
Derivative, fixed interest rate	2.79%
Interest Rate Swap 6 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Notional amount of interest rate derivatives	100,000,000
Derivative, fixed interest rate	2.79%
Interest Rate Swap 7 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Notional amount of interest rate derivatives	100,000,000
Derivative, fixed interest rate	2.80%
1-month LIBOR [Member] | Interest Rate Swap 8 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Derivative, Description of Variable Rate Basis	1-month LIBOR
1-month LIBOR [Member] | Interest Rate Swap 1 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Derivative, Description of Variable Rate Basis	1-month LIBOR
1-month LIBOR [Member] | Interest Rate Swap 1 [Member] | Notional Period 2 [Member]
Derivatives [Line Items]
Derivative, Description of Variable Rate Basis	1-month LIBOR
1-month LIBOR [Member] | Interest Rate Swap 1 [Member] | Notional Period 3 [Member]
Derivatives [Line Items]
Derivative, Description of Variable Rate Basis	1-month LIBOR
1-month LIBOR [Member] | Interest Rate Swap 1 [Member] | Notional Period 4 [Member]
Derivatives [Line Items]
Derivative, Description of Variable Rate Basis	1-month LIBOR
1-month LIBOR [Member] | Interest Rate Swap 2 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Derivative, Description of Variable Rate Basis	1-month LIBOR
1-month LIBOR [Member] | Interest Rate Swap 2 [Member] | Notional Period 2 [Member]
Derivatives [Line Items]
Derivative, Description of Variable Rate Basis	1-month LIBOR
1-month LIBOR [Member] | Interest Rate Swap 2 [Member] | Notional Period 3 [Member]
Derivatives [Line Items]
Derivative, Description of Variable Rate Basis	1-month LIBOR
1-month LIBOR [Member] | Interest Rate Swap 2 [Member] | Notional Period 4 [Member]
Derivatives [Line Items]
Derivative, Description of Variable Rate Basis	1-month LIBOR
1-month LIBOR [Member] | Interest Rate Swap 3 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Derivative, Description of Variable Rate Basis	1-month LIBOR
1-month LIBOR [Member] | Interest Rate Swap 3 [Member] | Notional Period 2 [Member]
Derivatives [Line Items]
Derivative, Description of Variable Rate Basis	1-month LIBOR
1-month LIBOR [Member] | Interest Rate Swap 3 [Member] | Notional Period 3 [Member]
Derivatives [Line Items]
Derivative, Description of Variable Rate Basis	1-month LIBOR
1-month LIBOR [Member] | Interest Rate Swap 3 [Member] | Notional Period 4 [Member]
Derivatives [Line Items]
Derivative, Description of Variable Rate Basis	1-month LIBOR
1-month LIBOR [Member] | Interest Rate Swap 4 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Derivative, Description of Variable Rate Basis	1-month LIBOR
3-month LIBOR [Member] | Interest Rate Swap 5 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Derivative, Description of Variable Rate Basis	3-month LIBOR
3-month LIBOR [Member] | Interest Rate Swap 6 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Derivative, Description of Variable Rate Basis	3-month LIBOR
3-month LIBOR [Member] | Interest Rate Swap 7 [Member] | Notional Period 1 [Member]
Derivatives [Line Items]
Derivative, Description of Variable Rate Basis	3-month LIBOR
Designated as Hedging Instrument [Member] | Interest rate swap agreements [Member]
Derivatives [Line Items]
Net loss on cash flow hedge reclassified from accumulated other comprehensive income	$ 720,000