Agreement of Limited Partnership (Details)		3 Months Ended								9 Months Ended
	Nov. 01, 2017 $ / shares	Dec. 31, 2017 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares	Mar. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($) $ / shares	Sep. 30, 2016 USD ($) $ / shares	Jun. 30, 2016 USD ($) $ / shares	Mar. 31, 2016 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / Unit shares	Sep. 30, 2016 USD ($)
Agreement of limited partnership
General Partners' Capital Account, Units Issued | shares			4,720							4,720
Declared and paid partnership distributions										$ 90,800,000	$ 208,500,000
Distribution Made to Limited and General Partners, Distributions Declared, Total			$ 42,700,000	$ 19,500,000	$ 28,600,000	$ 49,000,000	$ 78,800,000	$ 39,500,000	$ 90,200,000
Available cash received from operating partnership (as a percent)										99.00%
General Partner Distributions										$ 90,800,000	208,500,000
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 (per unit) | $ / Unit										0.605
Target Increment (per unit) | $ / Unit										0.605
Income and distribution allocation (as a percent)										100.00%
Minimum Quarterly Distributions | Maximum
Agreement of limited partnership
Incentive Distribution, Distribution										$ 11.3
First Target
Agreement of limited partnership
Target Limit (per unit) | $ / Unit										0.715
Target Increment (per unit) | $ / Unit										0.110
Income and distribution allocation (as a percent)										100.00%
First Target | Maximum
Agreement of limited partnership
Incentive Distribution, Distribution										$ 13.4
Second Target
Agreement of limited partnership
Target Limit (per unit) | $ / Unit										0.825
Target Increment (per unit) | $ / Unit										0.110
Income and distribution allocation (as a percent)										100.00%
Second Target | Maximum
Agreement of limited partnership
Incentive Distribution, Distribution										$ 15.4
Third Target [Member]
Agreement of limited partnership
Target Limit (per unit) | $ / Unit										1.045
Target Increment (per unit) | $ / Unit										0.220
Income and distribution allocation (as a percent)										100.00%
Third Target [Member] | Maximum
Agreement of limited partnership
Incentive Distribution, Distribution										$ 19.5
Final Target and Beyond
Agreement of limited partnership
Target Increment (per unit) | $ / Unit										0.000
Income and distribution allocation (as a percent)										100.00%
Final Target and Beyond | Minimum
Agreement of limited partnership
Target Limit (per unit) | $ / Unit										1.045
Incentive Distribution, Distribution										$ 19.5
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 and paid partnership distributions			$ 29,600,000	$ 17,900,000	$ 22,600,000	$ 32,700,000	$ 47,700,000	$ 28,000,000	$ 53,300,000
Cash distribution declared per common limited partnership unit (in dollars per unit) | $ / shares			$ 1.60	$ 0.97	$ 1.22	$ 1.77	$ 2.58	$ 1.51	$ 2.88
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 [Member]
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 and paid partnership distributions			$ 400,000	$ 200,000	$ 300,000	$ 500,000	$ 700,000	$ 400,000	$ 900,000
Cash distribution declared per common limited partnership unit (in dollars per unit) | $ / shares			$ 2.26	$ 1.04	$ 1.52	$ 2.60	$ 4.17	$ 2.09	$ 4.78
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 [Member]
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
Declared and paid partnership distributions			$ 12,700,000	$ 1,400,000	$ 5,700,000	$ 15,800,000	$ 30,400,000	$ 11,100,000	$ 36,000,000
Available cash distributed from operating partnership to General Partner (as a percent)										1.00%
General Partner Distributions			$ 400,000				$ 800,000			$ 900,000	$ 2,100,000
Cash distribution made, excluding when cumulative distribution is specified, (as a percent)										0.025%
Percentage of outstanding units owned by the General Partner and its affiliates			75.10%							75.10%
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 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 [Member]
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%
Subsequent Event
Agreement of limited partnership
Distribution Made to Limited and General Partners, Distributions Declared, Total		$ 33,800,000
Cash distribution declared per common limited partnership unit (in dollars per unit) | $ / shares	$ 1.36
Subsequent Event | Common Units
Agreement of limited partnership
Declared and paid partnership distributions		$ 25,100,000
Cash distribution declared per common limited partnership unit (in dollars per unit) | $ / shares		$ 1.36
Subsequent Event | Class B Common Units
Agreement of limited partnership
Declared and paid partnership distributions		$ 300,000
Cash distribution declared per common limited partnership unit (in dollars per unit) | $ / shares		$ 1.79
Subsequent Event | General Partner
Agreement of limited partnership
Declared and paid partnership distributions		$ 8,400,000