Common and Subordinated Units (Details) - USD ($)	6 Months Ended		25 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	May 24, 2019	Apr. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Nov. 05, 2018	Mar. 31, 2018	Dec. 31, 2017	May 26, 2017
Class of Stock [Line Items]
Partners' capital account, distribution amount per share (in dollars per share)						$ 1.35
Stock repurchase program, authorized amount								$ 75,000,000.0
Proceeds from issuance of common units, net of offering costs	$ (43,000)	$ 9,067,000
Series B Cumulative Convertible Preferred Units
Class of Stock [Line Items]
Preferred units minimum voting rights rate (percent)	15.00%		15.00%
Preferred units distribution rate	7.00%
Common Units
Class of Stock [Line Items]
Partners' capital account, units	205,956,000	105,494,000	205,956,000			109,377,000	108,363,000		104,926,000	103,456,000
Treasury stock, acquired (in shares)	136,665,000,000
Treasury stock, value, acquired, cost method	$ 2,200,000
Equity distribution agreement, maximum value											$ 100,000,000
Number of shares issued (in shares)	0	516,639
Proceeds from issuance of common units, net of offering costs		$ 9,100,000	$ 73,000,000.0
Common Units | November 2018 Repurchase Program
Class of Stock [Line Items]
Treasury stock, value, acquired, cost method	$ 4,200,000
Subordinated Units
Class of Stock [Line Items]
Partners' capital account, units					96,328,836
Common Units
Class of Stock [Line Items]
Partners' capital account, units				96,328,836