Agreement of Limited Partnership (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	0 Months Ended	9 Months Ended
	Nov. 04, 2013	Sep. 30, 2013	Sep. 30, 2012
Agreement of limited partnership
Partnership distributions paid		$ 406.8	$ 422.1
Available cash received from operating partnership (as a percent)		99.00%
Cash distribution declared per common limited partnership unit (in dollars per unit)	$ 2.02
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 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 and Class B Common Units
Agreement of limited partnership
Cash distribution made, excluding when cumulative distribution is specified, (as a percent)		99.975%
Common Units | Minimum Quarterly Distributions
Agreement of limited partnership
Income and distribution allocation (as a percent)		98.99%
Common Units | First Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		98.99%
Common Units | Second Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		85.859%
Common Units | Third Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		75.758%
Common Units | Final Target and Beyond
Agreement of limited partnership
Income and distribution allocation (as a percent)		50.505%
Class B Common Units | Minimum Quarterly Distributions
Agreement of limited partnership
Income and distribution allocation (as a percent)		0.985%
Class B Common Units | First Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		0.985%
Class B Common Units | Second Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		0.985%
Class B Common Units | Third Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		0.985%
Class B Common Units | Final Target and Beyond
Agreement of limited partnership
Income and distribution allocation (as a percent)		0.985%
General Partner
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 | 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
General Partner | Maximum
Agreement of limited partnership
Notice period for making decision to purchase the outstanding units		60 days
General Partner | Minimum Quarterly Distributions
Agreement of limited partnership
Income and distribution allocation (as a percent)		0.025%
General Partner | First Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		0.025%
General Partner | Second Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		13.156%
General Partner | Third Target
Agreement of limited partnership
Income and distribution allocation (as a percent)		23.257%
General Partner | Final Target and Beyond
Agreement of limited partnership
Income and distribution allocation (as a percent)		48.51%