Capital and Accumulated Other Comprehensive Income (Loss) (Redeemable Preferred Units) (Details) - USD ($) $ / shares in Units, $ in Thousands						12 Months Ended
	Sep. 14, 2017	Aug. 07, 2017	Aug. 24, 2015	Mar. 26, 2014	Apr. 09, 2013	Dec. 31, 2018	Dec. 31, 2017
Preferred Units [Line Items]
Liquidation preference						$ 1,266,250	$ 1,266,250
Units outstanding (in units)						50,650,000	50,650,000
Balance (in thousands, net of issuance costs) as of December 31,						$ 1,249,560	$ 1,249,560
Preferred stock, liquidation preference per share/unit (in dollars per unit)						$ 25.00	$ 25.00
Series C Preferred Units
Preferred Units [Line Items]
Liquidation preference						$ 201,250
Annual rate of dividend per share (in dollars per share)	$ 1.65625
Units outstanding (in units)						8,050,000	8,050,000
Balance (in thousands, net of issuance costs) as of December 31,						$ 219,250	$ 219,250
Preferred stock dividend rate	6.625%
Series G Preferred Units
Preferred Units [Line Items]
Liquidation preference						$ 250,000
Annual rate of dividend per share (in dollars per share)					$ 1.46875
Units outstanding (in units)						10,000,000	10,000,000
Balance (in thousands, net of issuance costs) as of December 31,						$ 241,468	$ 241,468
Preferred stock dividend rate					5.875%
Series H Preferred Units
Preferred Units [Line Items]
Liquidation preference						$ 365,000
Annual rate of dividend per share (in dollars per share)				$ 1.84375
Units outstanding (in units)						14,600,000	14,600,000
Balance (in thousands, net of issuance costs) as of December 31,						$ 353,290	$ 353,290
Preferred stock dividend rate				7.375%
Series I Preferred Units
Preferred Units [Line Items]
Liquidation preference						$ 250,000
Annual rate of dividend per share (in dollars per share)			$ 1.58750
Units outstanding (in units)						10,000,000	10,000,000
Balance (in thousands, net of issuance costs) as of December 31,						$ 242,012	$ 242,012
Preferred stock dividend rate			6.35%
Series J Preferred Units
Preferred Units [Line Items]
Liquidation preference						$ 200,000
Annual rate of dividend per share (in dollars per share)		$ 1.31250
Units outstanding (in units)						8,000,000	8,000,000
Balance (in thousands, net of issuance costs) as of December 31,						$ 193,540	$ 193,540
Preferred stock dividend rate		5.25%