Agreement of Limited Partnership (Details) | Nov. 01, 2017$ / shares | Dec. 31, 2017USD ($)$ / shares | Sep. 30, 2017USD ($)$ / sharesshares | Jun. 30, 2017USD ($)$ / shares | Mar. 31, 2017USD ($)$ / shares | Dec. 31, 2016USD ($)$ / shares | Sep. 30, 2016USD ($)$ / shares | Jun. 30, 2016USD ($)$ / shares | Mar. 31, 2016USD ($)$ / shares | Sep. 30, 2017USD ($)$ / Unitshares | Sep. 30, 2016USD ($) |
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 | |
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 | | | | | | | | | |