Agreement of Limited Partnership (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended	12 Months Ended										3 Months Ended								12 Months Ended					3 Months Ended								12 Months Ended					3 Months Ended								12 Months Ended
	Feb. 28, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Minimum Quarterly Distributions	Dec. 31, 2013 First Target	Dec. 31, 2013 Second Target	Dec. 31, 2013 Third Target	Dec. 31, 2013 Final Target and Beyond	Dec. 31, 2013 Final Target and Beyond Minimum	Dec. 31, 2013 Common and Class B Common Units	Dec. 31, 2013 Common Units	Sep. 30, 2013 Common Units	Jun. 30, 2013 Common Units	Mar. 31, 2013 Common Units	Dec. 31, 2012 Common Units	Sep. 30, 2012 Common Units	Jun. 30, 2012 Common Units	Mar. 31, 2012 Common Units	Dec. 31, 2013 Common Units Minimum Quarterly Distributions	Dec. 31, 2013 Common Units First Target	Dec. 31, 2013 Common Units Second Target	Dec. 31, 2013 Common Units Third Target	Dec. 31, 2013 Common Units Final Target and Beyond	Dec. 31, 2013 Class B Common Units	Sep. 30, 2013 Class B Common Units	Jun. 30, 2013 Class B Common Units	Mar. 31, 2013 Class B Common Units	Dec. 31, 2012 Class B Common Units	Sep. 30, 2012 Class B Common Units	Jun. 30, 2012 Class B Common Units	Mar. 31, 2012 Class B Common Units	Dec. 31, 2013 Class B Common Units Minimum Quarterly Distributions	Dec. 31, 2013 Class B Common Units First Target	Dec. 31, 2013 Class B Common Units Second Target	Dec. 31, 2013 Class B Common Units Third Target	Dec. 31, 2013 Class B Common Units Final Target and Beyond	Dec. 31, 2013 General Partner	Sep. 30, 2013 General Partner	Jun. 30, 2013 General Partner	Mar. 31, 2013 General Partner	Dec. 31, 2012 General Partner	Sep. 30, 2012 General Partner	Jun. 30, 2012 General Partner	Mar. 31, 2012 General Partner	Dec. 31, 2013 General Partner	Dec. 31, 2013 General Partner Minimum	Dec. 31, 2013 General Partner Maximum	Dec. 31, 2013 General Partner Minimum Quarterly Distributions	Dec. 31, 2013 General Partner First Target	Dec. 31, 2013 General Partner Second Target	Dec. 31, 2013 General Partner Third Target	Dec. 31, 2013 General Partner Final Target and Beyond
Agreement of limited partnership
Partnership distributions paid		$ 465.2	$ 558.3	$ 448.7
Available cash received from operating partnership (as a percent)		99.00%
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)											99.975%																																			0.025%
Cash distribution declared per common limited partnership unit (in dollars per unit)	$ 2.10
Target Limit					0.605	0.715	0.825	1.045		1.045
Target Increment					0.605	0.110	0.110	0.220
Income and distribution allocation (as a percent)					100.00%	100.00%	100.00%	100.00%	100.00%											98.99%	98.99%	85.859%	75.758%	50.505%									0.985%	0.985%	0.985%	0.985%	0.985%												0.025%	0.025%	13.156%	23.257%	48.51%
Partnership equity interest (as a percent)																																						1.00%								1.00%
Cash distributions paid												$ 37.4	$ 74.4	$ 86.6	$ 67.2	$ 76.4	$ 77.9	$ 74.0	$ 83.8						$ 0.6	$ 1.3	$ 1.5	$ 1.2	$ 1.3	$ 1.3	$ 1.3	$ 1.5						$ 20.4	$ 56.5	$ 68.6	$ 49.5	$ 58.5	$ 60.2	$ 56.2	$ 65.9
Cash distributions paid (in dollars per unit)												$ 2.02	$ 4.02	$ 4.68	$ 3.63	$ 4.12	$ 4.21	$ 4.00	$ 4.53						$ 3.09	$ 7.00	$ 8.31	$ 6.25	$ 7.21	$ 7.39	$ 6.96	$ 8.01
Percentage of outstanding units owned by the General Partner and its affiliates																																						75.30%								75.30%
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	60 days
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
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