Income Taxes (Federal Statutory Income Tax Rate to Income Before Income Taxes) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Income from operations before income taxes	$ 78,120,000	$ 114,424,000	$ 425,572,000
Partnership income not subject to federal income tax	(77,704,000)	(109,111,000)	(418,500,000)
Income subject to federal income taxes	416,000	5,313,000	7,072,000
Tax expense at federal statutory rate	146,000	1,860,000	2,475,000
State income taxes, net of federal tax	1,396,000	949,000	928,000
Return to provision, federal and state	(163,000)	(198,000)	(193,000)
Other	(68,000)	731,000	777,000
Remeasurement of deferred taxes due to enacted tax rate change	(5,270,000)	0	0
Income tax expense (benefit)	$ (3,959,000)	$ 3,342,000	$ 3,987,000
Effective tax rate on income from operations before income taxes	(5.00%)	3.00%	1.00%
Liability for uncertain tax positions	$ 0	$ 0	$ 0