Long-Term Incentive Plan (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended			80 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015
Long-Term Incentive Plan
Number of common units initially authorized for issuance under LTIP (in shares)	4,300,000		4,300,000
Vested (in shares)			(11,605)
Number of Non-vested Units
Outstanding non-vested units at the beginning of the period (in shares)			595,720
Vested (in shares)			(11,605)
Forfeited (in shares)			(14,953)
Outstanding non-vested units at the end of the period (in shares)	569,162		569,162		595,720			595,720
Weighted Average Grant Date Fair Value
Outstanding non-vested units at the beginning of the period (in dollars per share)			$ 38.85
Vested (in dollars per share)			35.76
Forfeited (in dollars per share)			32.82
Outstanding non-vested units at the end of the period (in dollars per share)	$ 39.08		$ 39.08		$ 38.85			$ 38.85
Repurchase Program
Aggregate common units authorized to be acquired (in shares)	1,242,427		1,242,427
Common units repurchased by General Partner (in shares)			0					838,505
Common units repurchased by General Partner								$ 24,800,000
Repurchased units not deemed outstanding	462,161		462,161		488,719			488,719
CEO Authorized LTIP
Long-Term Incentive Plan
Number of years for which Plan was approved			3 years
Aggregate amount of CEO authorized shares to grant in each calendar year			$ 2,000,000
Maximum value of grant			$ 550,000
CEO Authorized LTIP expiration period			6 years
Phantom Unit Awards [Member]
Long-Term Incentive Plan
Phantom units granted (in shares)	0		0		76,893	44,902	498,112
Compensation cost related to the non-vested awards not yet recognized	$ 10,800,000		$ 10,800,000
Number of Non-vested Units
Granted (in shares)	0		0		76,893	44,902	498,112
Phantom Unit Awards [Member] | Selling, general and administrative expenses
Long-Term Incentive Plan
Compensation expenses for phantom unit awards	$ 900,000	$ 1,100,000	$ 3,100,000	$ 3,200,000