Agreement of Limited Partnership (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	9 Months Ended
	Nov. 30, 2012 USDperunit	Sep. 30, 2012	Sep. 30, 2011
Agreement of limited partnership
Partnership distributions paid		$ 422.1	$ 318.7
Available cash received from operating partnership (as a percent)		99.00%
Cash distribution declared per common limited partnership unit (in dollars per unit)	4.12
Number of average trading days' closing prices used to determine the purchase price of outstanding units of non-affiliates		20 days
Number of days before the purchase announcement is made, as a basis for determining the purchase price of outstanding units of non-affiliates		5 days
Minimum
Agreement of limited partnership
Percentage of ownership of non-affiliates of the General Partner allowing majority owner to acquire outstanding units held by non-affiliated persons		25.00%
Notice period for making decision to purchase the outstanding units		30 days
Maximum
Agreement of limited partnership
Notice period for making decision to purchase the outstanding units		60 days
Minimum Quarterly Distributions
Agreement of limited partnership
Target Limit		0.605
Target Increment		0.605
Income and distribution allocation (as a percent)		100.00%
First Target
Agreement of limited partnership
Target Limit		0.715
Target Increment		0.110
Income and distribution allocation (as a percent)		100.00%
Second Target
Agreement of limited partnership
Target Limit		0.825
Target Increment		0.110
Income and distribution allocation (as a percent)		100.00%
Third Target
Agreement of limited partnership
Target Limit		1.045
Target Increment		0.220
Income and distribution allocation (as a percent)		100.00%
Final Target and Beyond
Agreement of limited partnership
Income and distribution allocation (as a percent)		100.00%
Final Target and Beyond | Minimum
Agreement of limited partnership
Target Limit		1.045
Common | Minimum Quarterly Distributions
Agreement of limited partnership
Income and distribution allocation (as a percent)		98.99%
Common | First Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		98.99%
Common | Second Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		85.859%
Common | Third Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		75.758%
Common | Final Target and Beyond
Agreement of limited partnership
Income and distribution allocation (as a percent)		50.505%
Class B Common
Agreement of limited partnership
Cash distribution made, excluding when cumulative distribution is specified, (as a percent)		99.975%
Class B Common | Minimum Quarterly Distributions
Agreement of limited partnership
Income and distribution allocation (as a percent)		0.985%
Class B Common | First Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		0.985%
Class B Common | Second Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		0.985%
Class B Common | Third Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		0.985%
Class B Common | Final Target and Beyond
Agreement of limited partnership
Income and distribution allocation (as a percent)		0.985%
General Partner's Interest
Agreement of limited partnership
Available cash distributed from operating partnership to General Partner (as a percent)		1.00%
Cash distribution made, excluding when cumulative distribution is specified, (as a percent)		0.025%
Partnership equity interest (as a percent)		1.00%
Percentage of outstanding units owned by the General Partner and its affiliates		75.30%
Period within which highest price is paid for any unit preceding the date the purchase is announced used to determine the purchase price of outstanding units of non-affiliates		90 days
General Partner's Interest | Minimum Quarterly Distributions
Agreement of limited partnership
Income and distribution allocation (as a percent)		0.025%
General Partner's Interest | First Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		0.025%
General Partner's Interest | Second Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		13.156%
General Partner's Interest | Third Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		23.257%
General Partner's Interest | Final Target and Beyond
Agreement of limited partnership
Income and distribution allocation (as a percent)		48.51%