Equity - Nonvested Restricted Stock Activity (Details) - Restricted Stock - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Restricted Stock Shares
Beginning balance (in shares)	6,435,871	6,271,175	5,354,975
Granted (in shares)	3,274,358	3,251,017	3,457,590
Vested (in shares)	(3,508,814)	(2,640,640)	(2,182,805)
Forfeited (in shares)	(448,211)	(445,681)	(358,585)
Ending balance (in shares)	5,753,204	6,435,871	6,271,175
Weighted Average Grant-Date Fair Value per Share
Weighted average beginning balance (in dollars per share)	$ 45.86	$ 39.99	$ 35.36
Granted (in dollars per share)	57.61	50.06	42.09
Vested (in dollars per share)	44.64	38.05	31.98
Forfeited (in dollars per share)	52.38	45.51	33.90
Weighted average ending balance (in dollars per share)	$ 52.78	$ 45.86	$ 39.99