Partners' Capital, Equity and Incentive Compensation Plans (Schedule of Quarterly Cash Dividends) (Details) - USD ($) $ / shares in Units, $ in Thousands						1 Months Ended	3 Months Ended									4 Months Ended	12 Months Ended
	Feb. 22, 2019	Jan. 15, 2019	Jan. 08, 2019	Jun. 25, 2018	Mar. 15, 2018	Apr. 14, 2018	Jan. 14, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Oct. 14, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Record Date												Sep. 22, 2017	Jun. 16, 2017	Mar. 16, 2017	Dec. 16, 2016
Payment Date												Oct. 05, 2017	Jul. 06, 2017	Apr. 05, 2017	Jan. 05, 2017
Per Share and Per Unit Rate												$ 0.39	$ 0.39	$ 0.39	$ 0.36			$ 1.53
Dividend/Distribution Amount												$ 22,200	$ 21,600	$ 21,400	$ 19,700			$ 84,900
Preferred stock, issued					4,280,000
Preferred stock issuance proceeds, net of costs																	$ 407,477
Series A Redeemable Perpetual Preferred [Member]
Record Date								Sep. 28, 2018	Jun. 29, 2018	Apr. 05, 2018
Payment Date								Oct. 15, 2018	Jul. 16, 2018	Apr. 16, 2018
Per Share and Per Unit Rate								$ 0.45	$ 0.45	$ 0.15
Dividend/Distribution Amount								$ 1,900	$ 1,900	$ 600
Preferred stock dividends per share cash paid						$ 0.14844
Dividend rate (as a percent)					7.125%												7.125%
Preferred stock, issued					4,280,000			4,280,000				0					4,280,000	0
Preferred stock issuance proceeds, net of costs					$ 103,200
Preferred stock redemption price per share								$ 25.00									$ 25.00
Threshold period of redemption of preferred stock																	120 days
Ownership interest								50.00%									50.00%
Convertible preferred stock par value								$ 0.01									$ 0.01
Preferred stock, liquidation preference					$ 25.00			25.00				$ 25.00					25.00	$ 25.00
Share cap price								$ 1.46929									$ 1.46929
Series B Convertible Preferred Stock [Member]
Record Date								Sep. 30, 2018
Payment Date								Oct. 15, 2018
Per Share and Per Unit Rate								$ 1.99
Dividend/Distribution Amount								$ 6,300									$ 6,300
Preferred stock dividends per share cash paid																$ 1.9861111
Dividend rate (as a percent)				6.50%													6.50%
Preferred stock, issued				3,162,500				3,162,500				0					3,162,500	0
Preferred stock issuance proceeds, net of costs				$ 304,000
Number of shares of the company's common stock issued upon initial conversion (in shares)								2.1264									2.1264
Minimum percentage of closing sale price of common stock under preferred stock conversion (as a percent)																	150.00%
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
Preferred stock, liquidation preference				$ 100.00				$ 100.00				$ 100.00					$ 100.00	$ 100.00
Share cap price								$ 5.1020									$ 5.1020
Subsequent Event
Record Date	Mar. 20, 2019
Payment Date	Apr. 04, 2019
Subsequent Event | Series A Redeemable Perpetual Preferred [Member]
Payment Date		Jan. 15, 2019
Per Share and Per Unit Rate		$ 0.45
Subsequent Event | Series B Convertible Preferred Stock [Member]
Payment Date		Jan. 15, 2019
Per Share and Per Unit Rate							$ 1.63
Subsequent Event | Series B Preferred Stock
Record Date	Mar. 31, 2019
Payment Date	Apr. 15, 2019
Underwriter's Option | Series A Redeemable Perpetual Preferred [Member]
Preferred stock, issued					280,000
Underwriter's Option | Series B Convertible Preferred Stock [Member]
Preferred stock, issued				412,500
At Market | Class A Common Stock
Maximum value of stock which may be issued															$ 300,000
Net proceeds from stock offering (in shares)																	0
Common stock
Record Date								Sep. 20, 2018	Jun. 20, 2018	Mar. 22, 2018	Dec. 05, 2017
Payment Date								Oct. 04, 2018	Jul. 06, 2018	Apr. 05, 2018	Jan. 05, 2018
Per Share and Per Unit Rate								$ 0.41	$ 0.41	$ 0.41	$ 0.39						$ 1.62
Dividend/Distribution Amount								$ 23,700	$ 23,700	$ 23,700	$ 22,200						$ 93,300
Net proceeds from stock offering (in shares)																		2,036,000	6,325,000
Common stock | Series A Redeemable Perpetual Preferred [Member]
Per Share and Per Unit Rate																	$ 1.04
Dividend/Distribution Amount																	$ 4,400
Common stock | Subsequent Event
Payment Date			Jan. 08, 2019
Per Share and Per Unit Rate			$ 0.41
Preferred stock | Series A Redeemable Perpetual Preferred [Member]
Net proceeds from stock offering (in shares)																	4,280,000