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	9 Months Ended
	Oct. 15, 2018	Oct. 04, 2018	Sep. 20, 2018	Jun. 25, 2018	Mar. 15, 2018	Apr. 14, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Oct. 14, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Record Date			Sep. 20, 2018								Jun. 16, 2017	Mar. 16, 2017	Dec. 16, 2016
Payment Date											Jul. 06, 2017	Apr. 05, 2017	Jan. 05, 2017
Per Common Share and Per Unit Rate											$ 0.39	$ 0.39	$ 0.36
Dividend/Distribution Amount											$ 21,600	$ 21,400	$ 19,700			$ 62,700
Preferred stock issuance proceeds, net of costs															$ 407,616
7.125% Series A cumulative redeemable perpetual preferred stock: $0.01 par value (liquidation preference $25.00 per share), 4,600,000 shares authorized, 4,280,000 shares issued and outstanding as of September 30, 2018; zero shares authorized, issued and outstanding as of December 31, 2017
Preferred stock, issued					4,280,000
6.50% Series B cumulative convertible perpetual preferred units: $0.01 par value (liquidation preference $100.00 per share), 3,162,500 units authorized, issued and outstanding as of September 30, 2018; zero units authorized, issued and outstanding as of December 31, 2017
Preferred stock, issued				3,162,500
Series A Preferred Stock
Preferred stock dividends per share cash paid						$ 0.14844
Dividend rate (as a percent)					7.125%
Preferred stock, issued					4,280,000			4,280,000							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
Share cap price								$ 1.46929							$ 1.46929
Series B Preferred Stock
Dividend rate (as a percent)				6.50%
Preferred stock, issued				3,162,500				3,162,500							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
Share cap price								$ 5.1020							$ 5.1020
Subsequent Event | Series A Preferred Stock
Payment Date	Oct. 15, 2018
Preferred stock dividends per share cash paid	$ 0.45
Dividend rate (as a percent)	7.125%
Subsequent Event | Series B Preferred Stock
Payment Date	Oct. 15, 2018
Preferred stock dividends per share cash paid														$ 1.9861111
Dividend rate (as a percent)	6.50%
Underwriter's Option | Series A Preferred Stock
Preferred stock, issued					280,000
Underwriter's Option | Series B Preferred Stock
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								Jun. 20, 2018	Mar. 22, 2018	Dec. 05, 2017
Payment Date								Jul. 06, 2018	Apr. 05, 2018	Jan. 05, 2018
Per Common Share and Per Unit Rate								$ 0.41	$ 0.41	$ 0.39
Dividend/Distribution Amount								$ 23,700	$ 23,700	$ 22,200					$ 69,600
Partnership distribution per unit							$ 0.39	$ 0.41
Common stock | Subsequent Event
Payment Date		Oct. 04, 2018
Per Common Share and Per Unit Rate		$ 0.41
Partnership distribution per unit		$ 0.41
Preferred stock
Record Date								Jun. 29, 2018	Apr. 05, 2018
Payment Date								Jul. 16, 2018	Apr. 16, 2018
Per Common Share and Per Unit Rate								$ 0.45	$ 0.15
Dividend/Distribution Amount								$ 1,900	$ 600						$ 2,500
Partnership distribution per unit								$ 0.15
Preferred stock | Series A Preferred Stock
Net proceeds from stock offering (in shares)															4,280,000
Preferred stock | Series B Preferred Stock
Net proceeds from stock offering (in shares)															3,163,000