Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified		6 Months Ended
	Jan. 18, 2012	Jun. 30, 2012 Interest Rate Swap Fixed Rate Paid 1.351% [Member]	Jun. 30, 2012 Interest Rate Swap Fixed Rate Paid 1.347% [Member]	Jun. 30, 2012 Interest Rate Swap Fixed Rate Paid 1.350% [Member]	Jun. 30, 2012 Interest Rate Swap Fixed Rate Paid 3.068% [Member]	Jun. 30, 2012 Interest Rate Swap Fixed Rate Paid 3.150% [Member]	Jun. 30, 2012 Interest Rate Swap Fixed Rate Paid 3.750% [Member]	Jun. 30, 2012 Interest Rate Swap Fixed Rate Paid 3.264% [Member]	Jun. 30, 2012 Interest Rate Swap Fixed Rate Paid 3.814% [Member]
Derivative instruments interest rate swap agreements
Derivative, effective date		Apr. 25, 2011	Apr. 25, 2011	Apr. 25, 2011	Jan. 25, 2011	Apr. 25, 2011	Jan. 25, 2011	Apr. 25, 2011	Jan. 25, 2011
Derivative, notional amount		$ 250.0	$ 250.0	$ 250.0	$ 1,000.0	$ 1,000.0	$ 1,000.0	$ 1,000.0	$ 1,000.0
Derivative, fixed interest rate	0.165%	1.351%	1.347%	1.35%	3.068%	3.15%	3.75%	3.264%	3.814%
Derivative, variable interest rate		0.245%	0.245%	0.245%	0.245%	0.245%	0.245%	0.245%	0.245%
Next reset date		Jul. 25, 2012	Jul. 25, 2012	Jul. 25, 2012	Jul. 25, 2012	Jul. 25, 2012	Jul. 25, 2012	Jul. 25, 2012	Jul. 25, 2012
Derivative, maturity date		Jan. 25, 2015	Jan. 25, 2015	Jan. 25, 2015	Jan. 25, 2015	Jan. 25, 2015	Jan. 25, 2015	Jan. 25, 2015	Jan. 25, 2015