Agreement of Limited Partnership (Details) $ / shares in Units, $ in Millions		3 Months Ended								12 Months Ended
	Feb. 08, 2016 $ / shares	Dec. 31, 2015 USD ($) $ / shares	Sep. 30, 2015 USD ($) $ / shares	Jun. 30, 2015 USD ($) $ / shares	Mar. 31, 2015 USD ($) $ / shares	Dec. 31, 2014 USD ($) $ / shares	Sep. 30, 2014 USD ($) $ / shares	Jun. 30, 2014 USD ($) $ / shares	Mar. 31, 2014 USD ($) $ / shares	Dec. 31, 2015 USD ($) $ / Unit	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Agreement of limited partnership
Payments of distributions to affiliates | $										$ (294.8)	$ (304.1)	$ (465.2)
Available cash received from operating partnership (as a percent)										99.00%
Total distributions declared | $		$ 87.5	$ 70.8	$ 60.4	$ 76.1	$ 49.4	$ 98.6	$ 95.0	$ 61.1
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
Target Increment										0.000
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
Agreement of limited partnership
Declared partnership distributions | $		$ 52.0	$ 43.6	$ 38.5	$ 46.3	$ 33.0	$ 57.5	$ 55.8	$ 38.8
Cash distribution declared per common limited partnership unit (in dollars per unit) | $ / shares		$ 2.81	$ 2.36	$ 2.08	$ 2.50	$ 1.78	$ 3.11	$ 3.01	$ 2.10
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
Agreement of limited partnership
Declared partnership distributions | $		$ 0.9	$ 0.7	$ 0.6	$ 0.7	$ 0.4	$ 1.0	$ 0.9	$ 0.6
Cash distribution declared per common limited partnership unit (in dollars per unit) | $ / shares		$ 4.64	$ 3.74	$ 3.20	$ 4.03	$ 2.61	$ 5.22	$ 5.03	$ 3.24
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%
Subsequent Event
Agreement of limited partnership
Cash distribution declared per common limited partnership unit (in dollars per unit) | $ / shares	$ 2.88
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%								1.00%
Declared partnership distributions | $		$ 34.6	$ 26.5	$ 21.3	$ 29.1	$ 16.0	$ 40.1	$ 38.3	$ 21.7
Percentage of outstanding units owned by the General Partner and its affiliates		75.30%								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%