Partners' Capital, Equity and Incentive Compensation Plans (Schedule of Quarterly Cash Dividends) (Details) - USD ($) $ / shares in Units, $ in Thousands																						3 Months Ended	12 Months Ended
	Jan. 15, 2021	Jan. 07, 2021	Sep. 30, 2020	Sep. 18, 2020	Jun. 30, 2020	Jun. 19, 2020	Mar. 31, 2020	Mar. 20, 2020	Dec. 31, 2019	Dec. 20, 2019	Sep. 30, 2019	Sep. 19, 2019	Jun. 30, 2019	Jun. 25, 2019	Mar. 31, 2019	Mar. 20, 2019	Dec. 31, 2018	Dec. 21, 2018	Jun. 25, 2018	Jun. 22, 2018	Mar. 15, 2018	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Dividend rate (as a percent)																					7.125%
Preferred stock, shares issued (in shares)																					4,280,000
Preferred stock issuance proceeds, net of costs																							$ 0	$ 0	$ 407,477
Preferred stock, liquidation preference (in dollars per share)																					$ 25.00
Series A Redeemable Perpetual Preferred
Dividend rate (as a percent)																							7.125%	7.125%
Preferred stock, shares issued (in shares)									4,280,000														4,280,000	4,280,000
Preferred stock, liquidation preference (in dollars per share)									$ 25.00														$ 25.00	$ 25.00
Series A Preferred Stock
Dividend paid to common stockholders (in dollars per share)			$ 0.45		$ 0.45		$ 0.45		$ 0.45		$ 0.45		$ 0.45		$ 0.45		$ 0.45
Aggregate Dividend/Distribution Amount (in millions)			$ 1,900		$ 1,900		$ 1,900		$ 1,900		$ 1,900		$ 1,900		$ 1,900		$ 1,900						$ 7,600	$ 7,600
Series B Preferred Stock
Dividend paid to common stockholders (in dollars per share)			$ 1.63		$ 1.63		$ 1.63		$ 1.63		$ 1.63		$ 1.63		$ 1.63		$ 1.63
Aggregate Dividend/Distribution Amount (in millions)			$ 5,100		$ 5,100		$ 5,100		$ 5,100		$ 5,100		$ 5,100		$ 5,100		$ 5,100						$ 20,400	20,400
Series A Redeemable Perpetual Preferred
Preferred stock redemption price per share																							$ 25.00
Threshold period of redemption of preferred stock																							120 days
Ownership interest																							50.00%
Convertible preferred stock par value																							$ 0.01
Preferred stock, liquidation preference (in dollars per share)																							25.00
Share cap price																							$ 1.46929
Series B Convertible preferred stock
Dividend rate (as a percent)																				6.50%
Minimum trading days of closing sale price of common stock under preferred stock conversion (in days)																							20 days
Maximum trading days of closing sale price of common stock under preferred stock conversion including the last trading day (in days)																							30 days
Series B Convertible preferred stock
Dividend rate (as a percent)																			6.50%
Preferred stock, shares issued (in shares)																			3,162,500
Minimum percentage of closing sale price of common stock under preferred stock conversion (as a percent)																							150.00%
Maximum trading days of closing sale price of common stock under preferred stock conversion including the last trading day (in days)																							30 days
Preferred stock, liquidation preference (in dollars per share)																			$ 100.00
Period of average daily volume weighted average price																							10 days
Share cap price																							$ 5.1020
Subsequent Event | Series A Redeemable Perpetual Preferred
Dividend paid to common stockholders (in dollars per share)	$ 0.45
Subsequent Event | Series B Convertible preferred stock
Dividend paid to common stockholders (in dollars per share)	$ 1.63
Underwriter's Option | Series A Redeemable Perpetual Preferred
Preferred stock, shares issued (in shares)																					280,000
Underwriter's Option | Series B Convertible preferred stock
Preferred stock, shares issued (in shares)																			412,500
Underwritten Offering | Class A Common Stock
Shares issued																						4,000,000
Common stock
Dividend paid to common stockholders (in dollars per share)				$ 0.47		$ 0.47		$ 0.47		$ 0.44		$ 0.44		$ 0.44		$ 0.44		$ 0.41
Aggregate Dividend/Distribution Amount (in millions)				$ 32,000		$ 31,500		$ 31,500		$ 28,600		$ 27,300		$ 27,300		$ 27,300		$ 23,700					$ 123,600	$ 105,600
Shares issued																								4,000,000
Common stock | Subsequent Event
Dividend paid to common stockholders (in dollars per share)		$ 0.47