Share-Based Compensation Plans - Summary of Restricted Stock Activity (Details) - Restricted Stock - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Non- Vested Shares
Non-vested shares, outstanding, beginning balance (in shares)	224,340
Number of shares granted (in shares)	2,375,584
Non-vested shares, vested (in shares)	(1,854,549)
Non-vested shares, forfeited (in shares)	(15,875)
Non-vested shares, outstanding, ending balance (in shares)	729,500	224,340
Weighted Average Fair Value
Weighted average fair value, outstanding, beginning balance (in dollars per share)	$ 81.61
Weighted average fair value, granted (in dollars per share)	65.12
Weighted Average Fair Value, Vested (in dollars per share)	66.31
Weighted Average Fair Value, Forfeited (in dollars per share)	78.12
Weighted average fair value, outstanding, ending balance (in dollars per share)	$ 66.86	$ 81.61
Aggregate Fair Value
Aggregate fair value, outstanding, beginning balance	$ 18,309,538
Aggregate fair value, granted	154,690,670
Aggregate fair value, vested	(122,983,162)	$ (5,100,000)	$ (3,800,000)
Aggregate fair value, forfeited	(1,240,174)
Aggregate fair value, outstanding, ending balance	$ 48,776,872	$ 18,309,538