Share and Unit-based Plans - LTIP Activity (Details) - LTIP units - $ / shares		12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Units
Balance at beginning of year (shares)		322,572	56,315	46,695
Granted (shares)		506,906	480,371	424,442
Vested (shares)	(55,934)	(192,846)	(214,114)	(414,822)
Forfeited (shares)		0	0	0
Balance at end of year (shares)	56,315	636,632	322,572	56,315
Weighted Average Grant Date Fair Value
Balance at beginning of year (in dollars per share)		$ 58.18	$ 73.24	$ 58.89
Granted (in dollars per share)		55.33	65.00	74.71
Vested (in dollars per share)		69.93	77.45	73.13
Forfeited (in dollars per share)		0.00	0.00
Balance at end of year (in dollars per share)	$ 73.24	$ 52.36	$ 58.18	$ 73.24