Share-Based Payments - Time-Vested Restricted Stock Units Activity (Details) - Time-vested restricted stock units - $ / shares		12 Months Ended
		Dec. 31, 2020		Dec. 31, 2019	Dec. 31, 2018
Shares
Beginning balance, unvested shares (in shares)		938,000
Granted (in shares)	[1]	620,000
Vested (in shares)		(440,000)
Forfeited (in shares)		(100,000)
Ending balance, unvested shares (in shares)		1,018,000		938,000
Weighted Average Grant Date Fair Value
Beginning balance, weighted average grant date fair value (in dollars per share)		$ 306.55
Granted (in dollars per share)		318.87	[1]	$ 304.44	$ 316.32
Vested (in dollars per share)		304.45
Forfeited (in dollars per share)		314.46
Ending balance, weighted average grant date fair value (in dollars per share)		$ 314.46		$ 306.55
Number of shares granted (in shares)	[1]	620,000
Director
Shares
Granted (in shares)	[1]	10,000
Weighted Average Grant Date Fair Value
Number of shares granted (in shares)	[1]	10,000

[1] RSUs granted in 2020 primarily represent RSUs granted in conjunction with our annual awards made in February 2020 and awards made in conjunction with the hiring of new employees. RSUs granted in 2020 also include approximately 10,000 RSUs granted to our Board of Directors.